Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dbishopbliss on April 23, 2021, 09:20:02 am
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In the planning stages. Here's a link to the schematic: Supro 1690T (https://el34world.com/charts/Schematics/files/Supro/Supro_1690t.pdf).
I'm trying to find a suitable power transformer now that ClassicTone is out of business. Based on the schematic (snippet posted below), I believe I should be looking for a 620VCT transformer. However, I have a number of NOS Mullard 5AR4 rectifier tubes that I would like to use instead of having to buy a 5V4G. From what I understand the 5AR4 has a lower voltage loss than the 5V4G.
Should I look for a transformer with a lower voltage? If so, how much?
If I went with a 5V4G (which I might do because its closer to the original and has more sag) then would the Hammond 290HBZ be suitable? Is 150mA enough for two 6L6s and three 12AX7s?
290HBZ (https://www.hammfg.com/part/290HBZ?referer=1063)GENERIC120V @ 60 Hz620V @ 150 mA50V Bias6.3Vct @ 2.25A5V @ 4A
(https://i.ibb.co/FWmH1d0/Supro-1690t-PT.png) (https://imgbb.com/)
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Trying to answer my own question... Looking at the 6L6GC datasheet I see the Max Signal Plate current is somewhere between 54 and 78mA per tube. I'm not sure of the exact voltages (schematic doesn't include them). Does this mean that if the plate is seeing 300V I should be ok with the 290HBZ, but if its 250V I should find something else?
(https://i.ibb.co/khMzD8p/6l6current.png) (https://imgbb.com/)
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Hammond 290HBZ should be fine, but expect the voltage to be higher than you expect. However, my choice would be Hammond 272FX.
https://www.hammfg.com/files/parts/pdf/272FX.pdf
Every new PT I've dealt with in the past 20 years put out more voltage than shown on the data sheets. That also includes several Hammonds.
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you have some control over plate volts and current with the bias resistor Rk. You also have some control with type of rectifier, and "dropping" stages in the PS. so as long as you're ballpark close. I've only done 1 build where I tried to get REAL close to datasheet, then when I wasn't happy with sound, i tweaked and the amp overall wound up +/- 30% from datasheet
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I just got a reply from Leon C on the gear page. He said he had 362V on the plates of the power tubes.
I see the 272FX is rated at 600 @ 115. Is the reason for the higher voltage due to higher mains closer to 120?
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the Hammond 272FX for the Power Transformer looks like a good choice. Now, on to finding an output transformer. This is where things get weird....
The schematic says the OT measured between 9K and 10K into a 1.6 ohm load. It also says that the transformer was underpowered. On the other hand, the S6420 Thunderbolt transformer was rated 35 watts, 5K into 8 Ohms. Leon C used the Thunderbolt transformer and his clone sounds pretty good to me.
I think I mentioned in another thread that I will probably go with the OT Transformer made by Trinity rated for 22 watts. It has a 5K and an 8K primary. I'm thinking I could wire a switch to go between the two primaries. This way, I could use the 5K which is like the Thunderbolt and the 8K which is closer to the value in the original.
Thoughts? Any other ideas for a good output transformer choice?
P.S., If I decide I really don't like the Trinity transformer then I could always use it to build a AC15 lite. :-D
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I have never seen inputs like this Supro before. I'm not exactly sure how to lay them out.
One of the odd things is that the "treble" inputs are not shorting jacks. Seems like this might be noisy. The signal is going through a small cap - I'm assuming this filters out the bass frequencies which is why its the treble input.
What is happening on the V1A tube? The schematic says there is a 6.8M resistor from grid to ground. I have never seen such a large resistor before in any circuit.
(https://i.ibb.co/y5vRY9H/Input-Schematic.png) (https://imgbb.com/)
Here the layout I have come up with. Mechanically, the connections are there (I think) but perhaps it could be improved upon. Let me know what you think.
(https://i.ibb.co/fSFH3Kz/Inputs-Layout.png) (https://ibb.co/RSbcXMk)
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I believe the 6.8M R is called grid leak bias? Notice there is no cathode R
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One of the odd things is that the "treble" inputs are not shorting jacks. Seems like this might be noisy. The signal is going through a small cap - I'm assuming this filters out the bass frequencies which is why its the treble input.
Correct. Not noisy. Notice the 100K to ground on the normal jack. Sunn did something similar with it's bright inputs. No shorting jacks were used. They just permanently connected a 150K from the grid to ground.
http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf
What is happening on the V1A tube? The schematic says there is a 6.8M resistor from grid to ground. I have never seen such a large resistor before in any circuit.
It's called grid leak bias. Look it up. Whenever you connect the cathode to ground you must apply a negative voltage to the grid for proper bias. You can do this using a fixed bias negative voltage applied to the grid or you can use grid leak bias. The easy way to identify a grid leak bias circuit is the cathode will be grounded and you will see a big resistor on the grid, usually accompanied with a coupling cap to prevent the negative grid voltage from leaking back to the previous stage.
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It's called grid leak bias. Look it up... The easy way to identify a grid leak bias circuit is the cathode will be grounded and you will see a big resistor on the grid, usually accompanied with a coupling cap to prevent the negative grid voltage from leaking back to the previous stage.
Got it! Any thoughts on the layout.
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the Hammond 272FX for the Power Transformer looks like a good choice.
I think I'm going to put the amp into a JTM 45 style chassis. It has the right number of cutouts for the tubes, inputs and controls. I just discovered that Mojo has a "Low Power" transformer for the JTM45 that is also 600VCT and is designed to fit in the chassis. Its back-ordered right now but I can wait.
The schematic says the OT measured between 9K and 10K into a 1.6 ohm load. It also says that the transformer was underpowered. On the other hand, the S6420 Thunderbolt transformer was rated 35 watts, 5K into 8 Ohms. Leon C used the Thunderbolt transformer and his clone sounds pretty good to me.
I think I mentioned in another thread that I will probably go with the OT Transformer made by Trinity rated for 22 watts. It has a 5K and an 8K primary. I'm thinking I could wire a switch to go between the two primaries. This way, I could use the 5K which is like the Thunderbolt and the 8K which is closer to the value in the original.
Trinity is not selling their OT by itself right now (supplies are limited because of covid and they don't want to break up sets). So I can either buy a set and save the PT for another project or start looking for alternatives. Hammond has a couple of options (of course) but the are fairly expensive.
Mojo has a couple of OT designed for other amps that could work:
- Tweed Super with 6K primary (same tubes as the 1690T)
- Trainwreck with 6.6K primary (30 watts)
- Supro 6424 with 7.5K primary (I think this is rated at 10 watts and is probably too low).
- Tweed Deluxe with 8.K primary
Thoughts?
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Layout looks fine. I thought you had already decided on transformers.
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Layout looks fine. I thought you had already decided on transformers.
I thought I had too. :-)
Because Trinity won’t sell their output transformer I am looking at other options. While I was looking I saw the low power version of the power transformer at mojo. Because it would drop right into a jtm45 chassis Without having to cut any additional holes I am reconsidering.
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I've been doing some reading about output transformers and I think I have a better understanding which can help me make a decision. Please let me know if I got this right.
The turn ratio of an output transformer doesn't change. If the schematic is correct, the original 1690T transformer has a turns ratio of about 75. This was calculated by taking the square root of 9000/1.6. So if the Page amp was running an 8 ohm speaker instead of the 1.6 ohm speaker this would mean the load would look like 45K = 75 * 75 * 8.
I plugged the numbers into an online calculator and the load line is very flat. Not sure if I'm reading the values correctly, but it says Output Power (W) at Max g1=5.32. Does that mean 5.32 watts? Maybe x2 and it will be 10.6 watts? I think I heard the the Page amp was only around 11 watts so maybe that makes sense.
I don't think I will be able to emulate that load without having something very custom made. But I did find the following transformers with a 8K primary and 4, 8 and 16 ohm secondaries. If I'm correct, then I could plug in a 16 ohm speaker to the 4 ohm secondary and get in the ball park (~32K load) and would be about 15 watts max.
https://www.mojotone.com/amp-parts/Output-Transformers/Tweed-Deluxe-Output-Transformer-w-4-8-16-OHM-Secondary (https://www.mojotone.com/amp-parts/Output-Transformers/Tweed-Deluxe-Output-Transformer-w-4-8-16-OHM-Secondary)
https://www.mojotone.com/amp-parts/Transformers_1/British-Style-18-Watt-Output-Transformer (https://www.mojotone.com/amp-parts/Transformers_1/British-Style-18-Watt-Output-Transformer)
https://www.edcorusa.com/cxpp25-ms-8k (https://www.edcorusa.com/cxpp25-ms-8k)
https://www.hammfg.com/part/1750PA?referer=1025 (https://www.hammfg.com/part/1750PA?referer=1025) (8.4K primary)
What do you think? Or am I over thinking this and I should just get a transformer designed for two 6L6s?
I guess the good news tweed deluxes and 18 watt marshalls are pretty popular so I could always try both and then sell the one I don't like.
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get a transformer designed for two 6L6s
transformers aren't made for tubes, they are made to couple AC from primary to secondary efficiently
tube at a specific "point" want a specific load to operate efficiently
none of that matters much in guitar amps :icon_biggrin:
if you're really geeked, match the transformer to the tube datasheet at operating point you want (ie 300vdc plate, 53mA PP)
get multiple outut taps. once it's working and you've played for a week through 7 different speakers and 3 guitars, then mess with speaker loads and repeat
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....square root of 9000/1.6. So if the Page amp was running an 8 ohm speaker instead of the 1.6 ohm speaker this would mean the load would look like 45K = 75 * 75 * 8.....
What is a "Page amp"?? First time that came up.
It is VERY likely that two 6L6 at reasonable voltage will be loaded in 5k to 10k, NOT 45k. As you noted, that's too high to suck any power.
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What is a "Page amp"?? First time that came up.
It is VERY likely that two 6L6 at reasonable voltage will be loaded in 5k to 10k, NOT 45k. As you noted, that's too high to suck any power.
Sorry, internal conversation. Supposedly Jimmy Page had a 1690T that got damaged so the two 10-inch, 3.2 ohm speakers were replaced with a 12-inch 8 ohm speaker. If my math is right (which I'm terrible at so it might not be), then the load in his amp would be 45K.
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two 10-inch, 3.2 ohm speakers
are you confident they weren't wired series for 6.4 ohms? 6.4 ohms and 8 ohms will "sound" nearly identical
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are you confident they weren't wired series for 6.4 ohms? 6.4 ohms and 8 ohms will "sound" nearly identical
I'm going off of what the schematic says.
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Questions about the tremolo circuit:
- I cannot find any 500K Linear pots. I assume an Audio pot will be fine. Unless someone has a source.
- The SPST 500K pots I can find are labeled "for guitar". Is there a reason I should not use a "guitar" pot?
- I'm not sure what is happening with the switch on the speed pot. Is it just an alternative to the foot switch? Could I just use a regular pot and only use the foot switch to turn things on/off?
(https://i.ibb.co/rb8sgdy/Supro-Tremolo.png) (https://imgbb.com/)
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Here you go:
https://www.mouser.com/ProductDetail/Bourns/PDA241-SRT01-504B0?qs=%2Fha2pyFaduhqlB3AUoaTVovzBtZT46J1paV7L%252BeUkfnAgKkVA0fWx%2FNOmraAQYXX
Hundreds in stock.
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Here you go:
https://www.mouser.com/ProductDetail/Bourns/PDA241-SRT01-504B0?qs=%2Fha2pyFaduhqlB3AUoaTVovzBtZT46J1paV7L%252BeUkfnAgKkVA0fWx%2FNOmraAQYXX (https://www.mouser.com/ProductDetail/Bourns/PDA241-SRT01-504B0?qs=%2Fha2pyFaduhqlB3AUoaTVovzBtZT46J1paV7L%252BeUkfnAgKkVA0fWx%2FNOmraAQYXX)
Hundreds in stock.
Thanks for that. I meant I cannot find a SPST 500K Linear pot.
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If that's the volume pot on the schematic, I don't see high voltage on it, and I bet a guitar pot would be fine.
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"Guitar pot" often means a different knob/shaft.
If you do not need an exact clone, get a pot and a separate switch. (This is better because you do not lose your setting when you turn off, but if you make a million, switch-on-pot can be a penny cheaper.)
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"Guitar pot" often means a different knob/shaft.
If you do not need an exact clone, get a pot and a separate switch. (This is better because you do not lose your setting when you turn off, but if you make a million, switch-on-pot can be a penny cheaper.)
Hmmm, I was thinking about using a JTM 45 chassis that a power switch and a standby switch. But the standby switch isn't really necessary so maybe I will use that for the tremolo.
On the other hand, maybe I won't bother with the switch and just use the foot switch.
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- I'm not sure what is happening with the switch on the speed pot. Is it just an alternative to the foot switch? Could I just use a regular pot and only use the foot switch to turn things on/off?
(https://i.ibb.co/rb8sgdy/Supro-Tremolo.png) (https://imgbb.com/)
Why are there two switches on the Tremolo? Is the one on the pot the "master" if you will. That is, if it is open, it doesn't matter what the foot switch does. If it is closed then you can turn the tremolo on/off with the foot switch. I'm not getting it.
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Don't worry. Be happy. Just build it as drawn. If there is no footswitch plugged in you still have a way to turn off the tremolo. If you want to use a footswitch just plug it in and the front panel switch must be on for the footswitch to operate. BTW, we can't see the full size pic on imgbb.com. Maybe you need to share the image or make it public or something. Much easier to just link to the schematic in Hoffman's library...
https://el34world.com/charts/Schematics/files/Supro/Supro_1690t.pdf
Or if you want an easier to understand switch arrangement, change to the S-6424 schematic...
http://sluckeyamps.com/supro/supro.pdf
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Just build it as drawn. If there is no footswitch plugged in you still have a way to turn off the tremolo.
I'm thinking about omitting the foot switch altogether and just have a Tremolo switch. I don't use tremolo very often and it wouldn't be a problem for me to just flip a switch on the amp.
If I am understanding things properly, with the switch closed and no foot switch connected, the cathode of V1B is connected to the cathode of V2A (I know the pin labels are off, it should really be labeled V2B). In this case, the tubes share cathode resistor R12 and the cathode bypass cap.
When the switch is open, then the cathode of V2A goes to ground through C34, but also through R35 before getting to the R12 cathode.
BTW, we can't see the full size pic on imgbb.com. Maybe you need to share the image or make it public or something.
I'm seeing the full size pic inline. That is why I have been using imgbb instead of trying to scale down and add as an attachment. But, when I click on the image it takes me to the imbb.com site. I thought it would show the same image but it doesn't. Hmm.
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Component layout question...
I am pretty sure I have all the components laid out according to the schematic. I have not included all the connections for the OT, heaters, indicator lights, grounds, etc. Before I finish all of that up I would appreciate some general feedback on the layout of the components.
If you look at the board from Left to Right I have the following:
- Shared cathode resistor and bypass caps (for A and B)
- Filter caps and voltage dropping resistors
- Phase inverter components
- Tremolo components
- Filter cap for C
- Tremolo channel preamp components
- Normal channel preamp components
The biggest question I have is regarding the placement of the Filter Cap for C location. I think I have seen Sluckey layouts where it is closer to the preamp section so that is what inspired the current layout but honestly I'm not sure what are the pros/cons of doing it this way.
Also, the diagram is to scale using a pre-punched JTM45 chassis. There are two light blue circles under the cathode resistors/filter caps area of the board where the OT wires will be routed. Any issues with this?
Your feedback is appreciated.
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Layout looks fine to me. I didn't check for accuracy.
Filter cap C location is fine. That 10µF cap is filtering all 6 of the little triodes. Bad idea IMO. Hope that works out well.
OT wire routing looks fine.
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That 10µF cap is filtering all 6 of the little triodes. Bad idea IMO. Hope that works out well.
I am following the original schematic. Why is it a bad idea? What would you do to improve it?
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I know I'm a little late on this but did you ever get the amp completed?
Was your layout good or do you wish you changed it around a bit?
I'm interested in doing a 1690T as well. I've done a 1624T and a S6606 successfully.
Thanks in advance
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I'm on hold for a bit... I bought a JTM45 chassis and I want to use the Low Voltage JTM45 Transformer (300V) from Mojo. They are built by Heyboer but they are way behind in manufacturing and don't expect to be caught up until July or August. I need to go over the schematic again to be extra sure that the layout matches.
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Filter cap C location is fine. That 10µF cap is filtering all 6 of the little triodes. Bad idea IMO. Hope that works out well.
I asked before but you probably missed it... What would you do differently? Have additional filter stages for those triodes?
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Don't know what I was thinking. I just noticed my S-6424 only has 3 filter nodes too and feeds 6 triodes from node C just like the 1690T. I never had any problems with my amp. And the Fender AB763 reverb amps feed 6 triodes from the same filter node. You'll be fine just staying with the schematic. Just ignore my previous comment. :icon_biggrin:
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...That 10µF cap is filtering all 6 of the little triodes....
It's situational. Six stages straight cascade could never be stable. But Supro has two chains, One is 3 stages but the last (the inverter) has major attenuation at grid. The other is a 4-cascade but attenuation and bass-cut between, plus mix-loss where the two channels comes together.
I would not care to bet one way or the other. Especially if somebody "mods" the losses. Me, for one-off work, I might plan to have one more B+ mode (space, lugs) just in case. Which is a cost-item if you build thousands, so Supro may have tinkered it to work.
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I just got my power transformer in so it's time to get back to the build. As a reminder, I'm using a pre-drilled JTM45 chassis. I realized the board I designed is 1 inch longer than a traditional JTM45 board. I can probably make it fit but it will be tight.
As an alternative, maybe I could install a multi-section capacitor similar to what a JTM45 uses. There is a 20/10/10/10 cap available that would allow me to have a separate cap for the two chains PRR mentions. Is there a downside to running multiple wires from the cap to the board?
On another build Sluckey recommended I put the last filter cap closer to the socket instead of running a long wire from the cap to the socket. I could use the multi-cap for the first two stages and put the caps for the last two stages on the board.
What do you think? Your input is appreciated.
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I could use the multi-cap for the first two stages and put the caps for the last two stages on the board.
I like that plan...
http://sluckeyamps.com/dual_lite/dual_lite.pdf
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The original schematic calls for the first stages to have 20uF and 10uF. I don't think a multi-cap exists with those values. I found the following:
- 30uF/20uF 475V
- 32uF/32uF 500V
- 16uf/16uF 500V
- 20uf/10uf/10uF/10uF 450V (I could leave two sections unused)
- 10uf/10uF/10uF/10uF 450V (I could connect two of the sections in parallel and leave one unused)
What difference would I expect/detect using one of the first two options?
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- 30uF/20uF 475V
- 32uF/32uF 500V
I would consider either an upgrade/improvement
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I would use the 32/32.
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The original circuit has three stages of filtering with 10K resistors between each stage. The last stage, "C", is providing voltage to the 2 preamp gain stages, the tremolo stage and the phase inverter (not sure if my terminology is correct). Basically, V1, V2 and V3.
If I introduce an additional stage of filtering, should I just add another 1K resistor and 10uF capacitor? Something like the attachment. I realize that will drop the voltage to V1, V2 a little but probably not by much.
V1 and V2 are used for the preamp and tremolo. V3 is the phase inverter. Would it be better to group V2 and V3 together?
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I suggest you just stick to the original schematic.
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I reworked my layout to use a multi-cap. I need to go back through for a final check to make sure all values, connections, etc are correct. But aside from that, please provide feedback on the layout. I have questions about the following specifically:
- Does it matter if I go from the second filter cap B to the output tube sockets pin 4 and then to the 10K resistor feeding C. Or, is it better to go from the filter cap to the 10K resistor and then to the sockets?
- As previously noted, Cap C is doing a lot of filtering. I moved it further to the right of the board. I could probably move it over even further to positions C' or even C''. Is there any advantage to doing that?
- I attempted to make most capacitors run vertically - mostly because this is what I see on most layouts. Sometimes this requires I use a jumper running at an angle. I had a number of capacitors running horizontally on one iteration of the layout. Is there a reason to avoid horizontal components.
- The board is 10.25" inches which is the same size found in the typical JTM45 layout. I have some extra room after going to the multi-cap. Should I spread things out more? Make the board smaller? Leave room for a D filter cap to experiment with?
If anyone has any other suggestion please chime in. This is my first attempt at laying out a circuit from scratch - Sluckey style.
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There are a few wiring errors in the tremolo circuit. I'll let you untangle them before I point them out. :icon_biggrin:
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There are a few wiring errors in the tremolo circuit. I'll let you untangle them before I point them out. :icon_biggrin:
Because I have a Mac now I can no longer use Visio (unless I pay for the professional version) and have been using a tool called OmniGraffle. OG is ok, but a little awkward. I decided to try DIYLC again - I don't remember what my issue was, but it wasn't great the last time I used it. However, I'm finding it much better and easier to use now. Plus, its easy to scale the components to the exact size so I'm re-doing the layout again. Hopefully, I will find the errors.
Do you have a rule of thumb/best practice about board size and location? That is... it seems the traditional board size for a JTM45 is 10.25 inches. From the pictures I have seen, it looks like the traditional board allows for the OT wiring to be routed without having to be under the board. However, there is room in my chassis for a 11.75 inch board - this would let me put all the caps on the board. But the filter caps would be under the OT. Is there a reason to avoid this?
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I like your layout just as is. It's neat and logical. I don't see any reason to redo it. I untangled the tremolo circuit. In doing so, I swapped positions with the speed pot and the 100K on the schematic and layout. Take a look and tell me what you think...
PS... You forgot to connect the 6L6 grids to the board.
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Thanks for going through the tremolo circuit. The good news is that when I redrew the circuit in DIYLC I came up with the same layout as you except for one... I had a missing 100K resistor. As I mentioned, I redrew the layout with all the filter caps on the board. The main reason I want to do this is because I already have the capacitors. Its not like a dual cap is that expensive, but I kind of like it with all the components on the board. :-) I have not included all the ground wires, and OT wires yet - those are coming.
I have two things I'm confused about. The first is the tremolo switch. The schematic says there is a switch on the Speed Pot and the Footswitch. Do I really need both? I really don't use Tremolo that much so I would be happy to user either a switched pot or a just a switch on the amp. I wouldn't use a footswitch.
The second thing I'm confused about is the Standby switch. On the schematic there is a note that says, "on power switch." What does that that mean? Are they saying that there was a DPDT switch and that when you turn on the amp the standby comes on... what would be the point of that? Or maybe its supposed to be a 3 position switch? Is there such a thing where the first position would turn on power and the second position would turn on (or is it off) the standby? Finally, the location of the standby is unlike anything I have seen before. It looks like it is connecting the grids of the power tubes. How does that work? Is is connecting each side of the phase inverter so signals cancel each other out?
Finally, there are no screen resistors on the power tubes. I'm thinking maybe I should add those.
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I have two things I'm confused about. The first is the tremolo switch. The schematic says there is a switch on the Speed Pot and the Footswitch. Do I really need both? I really don't use Tremolo that much so I would be happy to user either a switched pot or a just a switch on the amp. I wouldn't use a footswitch.
The switch on the FS jack is the third lug. It enables you to turn the tremolo on/off with the switch on the speed pot ***OR*** with the footswitch. I consider it essential, but you can easily just omit the FS jack if you want to.
The second thing I'm confused about is the Standby switch. On the schematic there is a note that says, "on power switch." What does that that mean? Are they saying that there was a DPDT switch and that when you turn on the amp the standby comes on... what would be the point of that? Or maybe its supposed to be a 3 position switch? Is there such a thing where the first position would turn on power and the second position would turn on (or is it off) the standby? Finally, the location of the standby is unlike anything I have seen before. It looks like it is connecting the grids of the power tubes. How does that work? Is is connecting each side of the phase inverter so signals cancel each other out?
The power switch is a three position switch. Could be an ON-OFF-ON DPDT. Or, it was popular with a lot of amps from that era to use a 2 pole 3 position rotary switch.
Finally, there are no screen resistors on the power tubes. I'm thinking maybe I should add those.
Your call. Tubes will probably last a bit longer. Might affect that "Page" voodoo! :l2:
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I was looking at a photo of a vintage 1690T when I noticed that the foot switch for the tremolo was located between the speed and intensity pots. Using that as a guide, the layout for the components became more obvious. The JTM45 chassis 6 holes for potentiometers. I am going to enlarge one of them and install the foot switch there.
I also noticed that the vintage 1690T must have used a DPDT on-off-on switch as suggested. The vintage switch was label standby (down) and on (up). Did they really have people turn the switch down for the heaters and then up, briefly turning everything off, to turn everything on? Seems very strange. I think I'm going to either go with a single switch - everything on - or two different switches.
On more thing, I decided to use a 47uF and 16uF filter caps instead of 20uF and 10uF as shown on the original schematic.
Other than the mains, heaters, and pilot light wiring I think I have the layout complete.
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a single switch - everything on
STBY is an over-rated 20th century thing :laugh:
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STBY is an over-rated 20th century thing :laugh:
You should look at the schematic. Even pdf64 would approve. :icon_biggrin:
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:laugh:
that took a minute to find!!!!
I went to bed a 1AM!!
I'll concede defeat on that one :icon_biggrin:
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I'm missing the joke. :-/
But speaking of the standby. Was I correct in how/why it works in the Supro?
That is, according to Valve Wizard the "least bad way" to implement a standby switch for tube rectifiers is to place it after the first reservoir capacitor but before the output transformer.
But the Supro has nothing of the sort. The standby switch has one pole connected to the grid of V4 and the other connected to the grid of V5. My assumption is that when the switch is closed the out of phase signals are combined canceling each other so the amplifier will make no (or little) sound. I assume that leaving it out means exactly that... don't connect those grids to each other.
I assume pdf64 must be anti-standby switches. Perhaps that's the punchline... the standby on the Supro really isn't a standby, it's more of a mute switch, so even he would approve.
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You got it, it's a mute so it works for "us" NO-STBY crowd :icon_biggrin:
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Here's an idea I have just to be a little different... Any reason I should not use one of these push button switches with the LED for a combined power on/off and indicator?
https://www.parts-express.com/SPST-Latching-Stainless-Steel-Waterproof-Tamperproof-19mm-Push-Button-Switch-with-120V-Red-L-060-944
https://www.parts-express.com/SPDT-Latching-16mm-Stainless-Steel-Tamper-Waterproof-Raised-Push-Button-Switch-with-Blue-Rin-060-958
I used the blue one on my recent attenuator build and I really like the feel of it. Just a light touch to turn off/on but solid feeling. I was thinking I could connect the LED to the heaters with a bridge rectifier and a resistor. This way, when you turn on the amp, the ring would light up.
Thoughts?
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I can't think of a reason not to use one of these switches, though I wish I knew more about how they actually work. They seem to be rated appropriately for US mains voltages.
The two examples are different, of course. The red one's light is operated off of mains voltage, where the blue one's light is intended for low voltages and is a separate circuit. I like the simplicity of the red one, with no need for bringing the heater current back to the light.
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The red one's light is operated off of mains voltage... I like the simplicity of the red one, with no need for bringing the heater current back to the light.
I'm thinking the same thing. I guess I can buy one and connect everything before modifying the chassis. That's another nice thing about the red one, it takes a 3/4" hole instead of 16mm so I can use a step-bit to enlarge the hole for the power switch.
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NBD - New Board Day! Is that a thing?
I got the board. One thing I forgot to do (or maybe you cannot do) is include holes for mounting the board on standoffs. You can see the pencil marks where I will be drilling.
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Here's how the board will fit in the chassis.
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The chassis was drilled for an output transformer that has 4 bolts. I need to drill 2 additional holes to mount this transformer. It's supposed to be for a Tweed Super 5F4 which was a dual 6L6 powered amp around 20 to 30 watts. I believe the original 1690T output transformer was undersized so the breakup may be at higher volumes. But, both of the amps I currently use breakup very early (Vox AC15 and Silvertone 1482) so having some headroom might be nice.
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With regard to drilling mounting holes for your board:
It is likely that you will want to dismount your board at some point after you have installed components. Often, you can get to the screws outside of the chassis to do this, but sometimes you will want to dismount the board from its standoffs. Make sure that your screws are accessible without removing soldered components. One of your pencil marks is directly in line with a turret path . . .
Yes of course I've made this mistake before . . .
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I'm going to use the push-button on-off switch so I had to enlarge one of the hole to 3/4". I also enlarged the 5th hole from the left to accommodate the foot switch or the tremolo.
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I have the transformers mounted. You can only see one of the mounting bolts here, but they are pretty brass ones that I got from Valvestorm. It was cheaper for me to buy a JTM45 mounting kit than try to get individual screws, nuts, etc. When I moved 2 years ago I got rid of all that stuff.
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I should have checked this beforehand... those fancy brass bolts for the octal tube sockets are bigger than the holes drilled in the chassis.
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No worries, pulled out my step bit again and enlarged the holes. Now I have all the sockets installed.
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This is an accident waiting to happen. Because I am not using a dual can capacitor there is a large hole in the chassis that someone can easily reach into. Anyone know where I can get a plug to cover this up?
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This is an accident waiting to happen. Because I am not using a dual can capacitor there is a large hole in the chassis that someone can easily reach into. Anyone know where I can get a plug to cover this up?
Easiest thing to do would be to make a cover out of a little square of sheetmetal, or install a dual cap can and use it for the plate/screen node supply only
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If you scroll down on this page you'll see "Hole Blocker Plates." But, I'm not sure if the mounting holes match your chassis.
https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=MiscHardware&ORDER_ID=194341330
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If you scroll down on this page you'll see "Hole Blocker Plates." But, I'm not sure if the mounting holes match your chassis.
https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=MiscHardware&ORDER_ID=194341330 (https://hoffmanamps.com/MyStore/perlshop.cgi?action=template&thispage=MiscHardware&ORDER_ID=194341330)
That’s what I was looking for… I was looking on the sockets page. I can always drill new holes if they don’t line up.
I could also install a cap for looks. I was digging through my box of parts and found some old caps that I never threw out. :-)
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I don't think those will completely cover that big hole. I would send Doug a pm and ask him to measure them. He usually responds pretty quickly.
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I decided to go the cheap way out and use a 50/50 cap I had to cover the hole for now. I just discovered there is a "makerspace" 2 miles from me. It has all sorts of equipment including a metal shop, cnc machining and laser cutter. I can make my own plug in the future.
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Another hole that was the wrong size. I bought the fuse holder from Valvestorm while I was buying the other screws, nuts, spacers, etc. It requires a 5/8" hole while the chassis has a 9/16" hole. Which seems an odd size, but that is what my calipers said. One lesson learned... maybe its better to buy chassis hardware from the same place you buy the chassis.
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Everyone's least favorite part... I took the long road running the filament wires along the corners of the chassis. Because there are no signals flowing on that end I probably could have run the wires straight down towards the back of the amp.
I used the Valve Wizard method of wiring the filaments where you run the wire across the top of the socket. He claims any other method can induce hum.
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That's an odd looking fuse holder. What keeps it from turning when you replace a fuse?
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HT wires connected to the rectifier. I had some N4007 diodes sitting in my drawer so I decided to follow Rob Robinette's recommendation and install diodes from terminals 3 to 4 and 5 to 6 just in case the tube fails. I thought I had some UF4007 diodes, but I expect these will work just fine.
That's an odd looking fuse holder. What keeps it from turning when you replace a fuse?
There are metal "ears" on the side that seem to expand when you insert the fuse holder into the chassis. It is VERY tight.
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I installed the pots and jacks on the front panel to make sure they fit. Actually, I installed them on the back but then realized something... if I wire up the mains and fuse holder now then I will not be able to mount a rear plate without unsoldering them. Also, one of the speaker jacks is above one of the output tubes so it will be difficult to solder components to the socket if I install it now.
I need to start designing my face plates.
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Ground bus and jumpers wired up using Hoffman's lacing style. There will be components sitting on top of those jumpers so I'm thinking that I might move those to the back of the board.
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Speaking of the back of the board... I cut the insulation back far enough so the leads can go all the way through the turret and then wrap it over the top so it doesn't fall out.
By the way, Sluckey recommend the Kester No Clean flux to someone a while back. This is my first time using it... nice stuff. I love that it doesn't get all sticky like my old flux.
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I started to dry fit components on the board and I realized a couple of things... First, I am going to have a very hard time installing the nuts to secure the turret board to the chassis. Second, some of the components are very close together which is going to make soldering cables very challenging.
I decided to same myself the aggravation and rework the board without the first two filter caps. I'm going to use a dual filter cap mounted to the chassis (fixed my hole problem) and spread out the rest of the components more. In addition, I'm leaving extra space on the ends of the boards for the standoffs. Because the rest of the components are spread out more, there is plenty of room for the center standoffs as well.
I guess this is what you learn from experience. :BangHead: :cussing: :sad2:
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While I'm waiting for my new turret board to arrive I was doing a little thinking (which is dangerous).
According to the schematic, the primary of the original amp's OT was between 9K and 10K. The secondary was 1.6R (two 10" 3.2 ohm speakers in parallel). For easy math, let's say the primary was 9600 / 1.6 for a ratio of 6000:1.
Reportedly, Jimmy Page's amp was damaged and the speakers were replaced with a single 12" 4 ohm speaker. If I understand how things work, this means that the amp would now see a 24K load. According to the universal load line calculator, this makes for a very flat load line and like 11 watts power. If I recall correctly, I read that the amp was around 10 watts and runs cold so I think that makes sense.
My output transformer is 6000 / 4 for a ratio of 1500:1. If I plug a 16 ohm speaker into the 4 ohm secondary then the amp will see a load of 24K just like Jimmy Page's amp.
Do I have that right?
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I got the new board in (black one). You can see that things are spread out more compared to the original board.
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Another shot of the new board in the chassis. Lots more room for standoffs in the corner as well as in the middle. Looks a little steam punk with the steel and brass.
The old holes in the chassis will be for... ventilation. Yeah, that's it. Those holes are to keep things cool.
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Reportedly, Jimmy Page's amp was damaged and the speakers were replaced with a single 12" 4 ohm speaker.
[...]
Do I have that right?
Well the 2x 10" speakers were replaced with a single Rola 12" 8 ohm speaker, not a 4 ohm one - you can verify directly by finding a back photo of the Sundragon Amp, it mounts a very nice 8 ohm Jensen P12Q.
I guess you have to re-elaborate your load consequently :wink:
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Well the 2x 10" speakers were replaced with a single Rola 12" 8 ohm speaker, not a 4 ohm one - you can verify directly by finding a back photo of the Sundragon Amp, it mounts a very nice 8 ohm Jensen P12Q.
I guess you have to re-elaborate your load consequently :wink:
Well... I have two 16 ohm speakers I can run in series if I want to try it out.
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Well... I have two 16 ohm speakers I can run in series if I want to try it out.
Worth a try, given that the 8 ohm load on 2 ohm OT is arguably the greatest "mod" on Page amp respect to the stock Coronado.
Incidentally, let me tell that IMHO the Sundragon demos I've watched aren't that attractive, the whole thing sounds grunted and lacking in bite to me - devil knows how Jimmy Page came up with that incredibile tone from it on Led Zeppelin I! :worthy1:
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Well... I have two 16 ohm speakers I can run in series if I want to try it out.
Worth a try, given that the 8 ohm load on 2 ohm OT is arguably the greatest "mod" on Page amp respect to the stock Coronado.
Incidentally, let me tell that IMHO the Sundragon demos I've watched aren't that attractive, the whole thing sounds grunted and lacking in bite to me - devil knows how Jimmy Page came up with that incredibile tone from it on Led Zeppelin I! :worthy1:
Who knows... probably EQ and compression in the board. :-)
That said, I think the videos that Leon C has put out of his Coronado clones and original amps with the same circuit sound great (although I wish he did not record with his "wet rig"). I emailed him and he said he used the ClassicTone # 40-18060 which is the "Thunderbolt" transformer. The specs on that are no longer posted but I recently found out it has a 5K primary and rated for 35 watts so I'm in the ballpark with the Tweed Super OT.
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I have the board almost fully loaded... somehow I'm missing one 0.047uF and one 0.0047uF capacitor on the right hand side. I also never submitted the order for the 6.8M resistor needed.
I used a different technique for installing the components this time. In the past I always put the leads into the holes of the turrets on the top. This time I wrapped the leads. I started on the left and got better as I moved right. I probably wrapped too much on those first components. That said, they have very strong mechanical connections. I could probably fire up the amp now without any solder and it would probably work (not going to try it).
In retrospect, I would not install the jumpers between turrets on the top of the board in advance. Things are getting a little crowded in places. Instead, I would wrap the lead around the turret and leave it long so it extends to the next turret.
I plan on installing all the leads to the sockets, pots, jacks, etc under the board. I will strip enough insulation back so the lead can go all the way through the turret and then bending over the top.
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I have a couple of other caps I could use but it will be hard to make them fit so I'm going to wait for the new ones to arrive. Interesting to compare the size of these vintage caps to the size of the modern sozo cap on the lower left hand side.
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.... I could probably fire up the amp now without any solder and it would probably work (not going to try it)....
Bogen did that once. ONE of the 100+ connections was crimped but the solderer missed it. It ran for years in heavy duty (driving a motor) until it caught fire and was thrown-out (I caught it).
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Which rectifier did you decide on? The 5V4G datasheets I looked at only show a max 32uF first filter cap. Your 47uF might cause it to arc. The 5AR4 can handle up to 60uF, so you would be fine there.
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Which rectifier did you decide on? The 5V4G datasheets I looked at only show a max 32uF first filter cap. Your 47uF might cause it to arc. The 5AR4 can handle up to 60uF, so you would be fine there.
I'm using a 32uF cap and I will probably be going with a 5AR4 because the voltage of my PT is lower than the original PT. I think things will work out because mains are higher and 5AR4 has lower voltage drop, but if the voltage is too high I figured I could swap in different rectifier tubes.
That said, I didn't realize filter caps had max values... The first several datasheets I looked at did not have a value listed at all. One of them says the *typical operation is 10uF. Then I noticed footnotes that say things like,
when filter condensers larger than 40uF are used, it may be necessary to add additional plate supply impedance
when a filter-input condenser larger than 40uF is used, it may be necessary to use more plate-supply impedance than the minimum value show to limit the peak plate current to the rated value.
How do I increase the plate-supply impedance? I don't know what that means.
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How do I increase the plate-supply impedance? I don't know what that means.
I like to use two power resistors (50, 100, 200Ω, etc. @5 watts), placed between the PT HT wires and pins 4 and 6 of the rectifier socket. Resistors can be mounted directly on the socket using spare pins.
However, I don't think this will be necessary with a 32µF filter cap at node A.
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I take everything about the JP amp with a grain of salt. For decades Jimmy claimed he could not remember the details of that amp. Then the opportunity to make money off a "historically accurate" clone of that amp jogged his memory. I'm not dissing this project as I love Valco amps. But the story of the Sundragon just does not add up for me. I agree with the comments about the studio EQ and such contributing to that tone. My favorite players are Ry Cooder and Richard Thompson - the only thing that will get me close to their sound/tone is practice. And at my advanced age; it ain't going to happen, but building amps is fun; so I continue.
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My favorite players are Ry Cooder and Richard Thompson
"Red hair and black leather, my favorite color scheme"
I've seen Thompson a few times. It's mesmerizing to watch him play.
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I take everything about the JP amp with a grain of salt. For decades Jimmy claimed he could not remember the details of that amp. Then the opportunity to make money off a "historically accurate" clone of that amp jogged his memory. I'm not dissing this project as I love Valco amps. But the story of the Sundragon just does not add up for me. I agree with the comments about the studio EQ and such contributing to that tone. My favorite players are Ry Cooder and Richard Thompson - the only thing that will get me close to their sound/tone is practice. And at my advanced age; it ain't going to happen, but building amps is fun; so I continue.
I'm actually not a huge Led Zeppelin fan. I knew the music because I grew up with it, but I was more into The Who. However, every time I listen to clips of Supro amps I really like their tone. Just so happens I liked Leon C's clips of the 1690T the best. Then I searched out others to confirm my preference of that model over other Supros so here we are.
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Leon's clips are great! Every time I listen to his clips I want to build the amp he is playing. He is a master of his crafts - amp building and guitar playing. My favorites are the older all octal Valcos, but this amp is also right up there. I have an original upper and lower chassis along with a stripped cabinet of this amp awaiting attention. The two chassis have suffered many indignities which is why I keep passing over the project.
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"Red hair and black leather, my favorite color scheme"
I've seen Thompson a few times. It's mesmerizing to watch him play
Yes it is. I have seen him more times (10+) than any other artist and will see him again in Boothbay Harbor, Maine in September. His playing is impeccable and he rapport with the audience is amazing. If any here are unaware - go to youtube and watch the various versions of 1952 Vincent Black Lighting which is the source of Dwinstonwood's quote. Acoustic and electric he is a master.
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I have the transformers mounted.
Do you know about the "headphone trick," and have you used it to verify that the way you have the transformers mounted will be the lowest noise way? If not, you might want to look into this before you finalize your circuit board and start hooking everything up.
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Do you know about the "headphone trick," and have you used it to verify that the way you have the transformers mounted will be the lowest noise way? If not, you might want to look into this before you finalize your circuit board and start hooking everything up.
Nope... I don't know the trick. The chassis is pre-drilled for a JTM45 so I'm not sure how much flexibility I have mounting the transformers. The PT is a lay down transformer so it cannot move. I could drill additional holes for the OT and re-orient it 90 degrees (or some other angle) or move it further away from the PT because there is no choke like the JTM45 has.
This is what I found
First set up the power transformer. If it's not installed yet, all the better, but if it is, this will still help. If the PT is not installed, wire up the primaries to a power cord, but do not wire up any secondaries-- tape the ends of the secondaries. If the PT is installed, fine, just make sure there is no circuit on any of the secondary leads (pull all tubes, and disconnect leads and tape them as necessary).Now set up the signal transformer (output, reverb, whatever). Hook one set of OT secondaries (your choice, but I usually go from ground to the highest impedance tap, don't worry about impedance matching here!) to a set of headphones. Tape the primary leads.Put the headphones on, plug in the power cord, and start moving the signal transformer around. Try it all over the chassis, and also rotate it. While you might expect the least hum with the transformers as far from each other as possible and at right angles (and that may be the case for you), it might come at an odd angle and/or position. Sometimes the least hum occurs when the transformers are side by side. When you find the quietest spot, use a permanent marker to mark where the mounting holes should be.
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I attached the primaries of the PT to a power cable and then soldered the OT secondaries to a jack so I could easily plug in a set of headphones. I plugged the power cable into a power strip with a switch so I could turn it on / off remotely.
The photo where the OT is lined up straight is the original location. Hummmmmmmmm. Not terrible but clearly audible.
The photo where the OT is at an angle is almost silent. I had my daughter with "dog hearing" listen for me to find the most quiet position. Moving the OT further away did not make any improvement. Time to drill another hole.
I have to admit, I was surprised by the results. I would have thought that after all these years of people building a JTM45 style amplifier that someone would have figured out that the amp is quieter with an angled OT. Or, is this just a function of this PT/OT combination?
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Most musicians don't have a keen-ear daughter on headphones.
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Most musicians don't have a keen-ear daughter on headphones.
At least one that is willing to come into the workshop and listen to buzzzzzz. Full disclosure... I had to drag her to come listen and there was some eye rolling. :rolleyes:
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:laugh:
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There really is such a thing as a too-strong magnifying glass.
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There really is such a thing as a too-strong magnifying glass.
Perhaps, but even my 55 year old ears could hear the difference.
Different topic, but sort of related. I downloaded the latest version of PSUD2. I am trying to model the power supply for the Supro 1690T so I can compare the differences between using the 20uF/10uF/10uF filter caps as designed and the 32uF/32uF/16uF filter caps that I will be using. However, I keep getting warnings saying, "A current sink has pulled the voltage below zero for more than 5 mains cycles at time..." I think either I am entering some values incorrectly or I am going to need to change the power supply.
Here is what I used:
Primary
Supply Voltage: 125.6 V (measured across the wall socket)
Winding Resistance: 1.7 Ohm (measured primaries end to end)
Secondary
Offload Voltage: 649 V (measured secondaries end to end, not the center tap)
Winding Resistance: 44.5 Ohm (measured end to end, not the center tap)
I read that you should halve the values for calculations when you have a CT. This calculated:
Turns Ratio: 1:2.58
Estimated Impedance: 33.59 Ohms
For the model I used a Full-wave rectifier followed by a C filter, RC filter RC filter, Constant Current Load (seen screen show below):
The datasheet for 6L6 tubes running push-pull class AB1 gave a value of 88mA and each 12AX7 draws 1.5mA for a total of 91.5mA (do I have that right?).
Using all these values, the simulation predicts the voltage at C2 would be around 213V and C3 would be like 18V. In a message from Leon C, the target for the plate of the 6L6 tubes is 362V. So, I think I'm off with something.
Are the values I plugging in to PSUD2 wrong or do I need to adjust the R1 and R2 values. Thanks for your help.
P.S. The screen shot says 91.5A, but it should say mA. Same thing happens.
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guessing the 6L6 gets it's "feed" from the C1/R1junction?
if so, that's where you want the high current, not at the end of the PS string
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guessing the 6L6 gets it's "feed" from the C1/R1junction?
if so, that's where you want the high current, not at the end of the PS string
I tried that… I still get the same results.
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:laugh:
my co-worker Paul loved using modeling software, me I just kept changing things til I found the broke one, I'd be 1/2way home n paul would call, "where'd you go?, I think it's......"
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I played with the values of the resistors and the following model no longer raises errors and the voltages are in the ballpark of what I think they should be.
R1 has a value of 560 Ohms - WAY down from 10K.
R2 has a value of 20K - double the 10K value.
I have also changed the rectifier to a 5AR4 - I have a couple of NOS Mullard 5AR4 tubes so why not use them?
Again, Leon C said the plate voltage on the 6L6 was around 362V - This model predicts 364V.
I have no idea what the voltages for the other tubes should be, but based on a schematic for a Supro 1624T I'm thinking around 270V after the last filter cap. The 1624T has a similar pre-amp section with the same tube complement and controls so I'm guessing that is probably a pretty good target. The model predicts 273V.
I could get similar results with a 5V4G rectifier, R1 = 270 ohms.
If my calculations are correct (which I never trust myself)... option 1 drops 52V across a 560R resistor so it would need to be 10W ~= (52*52/560) * 2. Whereas, the second option drops 26V across a 270R resistor so it would need to be 5W ~= (26*26/270) * 2.
Perhaps I didn't do anything wrong before. Maybe I just needed to adjust the values for my transformer. What do you think?
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the values sound right, I typically have ~~~470 ohms for my 1st R, which feeds the power tube, 2 more R's in the many k range
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And now, back to our regularly scheduled program… I got the faceplates in.
Years ago I would use the name Slackdog Industries when demoing software. I decided I liked that better than using my last name. Plus, now I get to use my dog’s names for the model.
Time to start soldering.
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Looks top notch! :thumbsup:
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Very nice! :icon_biggrin:
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I have a wire routing question. I don't think one way versus another would make a difference electronically, but as I found out with my Vox (and reading other's posts) I know that routing wires poorly can make a difference...
The option I see most often is running from the filter cap to the 10K resistor, then from the resistor to screen of the power tube.
For convenience, I would like to route the wire from the cap to pin 6 on V5, then to pin 6 on V4, then to the 10K resistor. There will be fewer wires crossing by doing this and the wires will be shorter. Any reason that I should not hook things up this way?
Note: not all wires are shown for clarity.
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Either way is fine but I prefer the second drawing.
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And now, back to our regularly scheduled program… I got the faceplates in.
Years ago I would use the name Slackdog Industries when demoing software. I decided I liked that better than using my last name. Plus, now I get to use my dog’s names for the model.
Time to start soldering.
Where (how) did you do the faceplate? I am building a Supro 1696TN slant cab and am looking to repro the original graphics
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Where (how) did you do the faceplate? I am build a Supro 1696TN slant cab and am looking to repro the original graphics
I designed the faceplate myself using Inkscape. It works pretty well and can read all sorts of file formats including Illustrator, dxf, etc. Mojo shared the dxf template for the JTM 45 chassis with me. I was able to convert that to a layer in the software for the cutouts. I created a new layer for my labels, logos, artwork around that. Then I saved the file as a PDF and sent it to Jeff at JT Productions (http://www.jtpro.com/ampfaceplate.html) (phone number is on the website).
The process was very simple and fast. I had faceplates made in the past and there was a lot more back and forth and corrections to the file required (missing fonts, line thickness, etc). I made the process much more difficult because I was worried about that stuff. It seems the new technology is pretty much like using a printer. Just send the PDF, Jeff will send you a version that shows where the cuts will be made and you're done.
If you are fabricating your own chassis without a laser cutter then the challenge will be getting the holes in the chassis to line up exactly with the holes in the faceplate. I ordered faceplates for my AC15 as well (they have to match), but I drilled the chassis by hand so I'm not expecting things to go as smoothly. I used a paper template when drilling the cutouts but it had to be printed out on multiple sheets of paper that were taped together.
If I were to do it again I would order a "pilot hole" template that would only have cutouts around 1/8". Then, mount the template on the chassis with tape. Drill one pilot hole and secure it with a screw. Then drill the rest of the pilot holes. After that, it would be easy to use a step drill bit to drill the rest of the holes to size. That would probably add around $20 to the project but if you are going through the hassle of reproducing the original graphics it will probably be worth the extra $$$.
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It lives! But its only half alive... I say half alive because the voltages seem to be about half of what I think they should be. I was expecting the following:
| Point | | Expected | | Measured |
| PT Secondaries | | 300 VAC | | 316 VAC |
| Rectifier Pin 4 and 6 | | see comment below | | 146 VDC |
| C1 | | 388 VDC | | 197 VDC |
| C2 | | 308 VDC | | 175 VDC |
| C3 | | 278 VDC | | 160 VDC |
| V4/V5 Plate | | 385 VDC | | 192 VDC |
The Plate Voltages were given to me by Leon C so I'm not sure how accurate they will be for my build. The other voltages were based on the PSUD model. Even if they aren't exact, I would expect them to be in the same general ballpark and they seem way off to me.
One thing I did, but I have never done before was install "backup diodes" on the rectifier socket (https://robrobinette.com/5e3_Modifications.htm#Backup_Diodes) as described by Rob Robinette. They are going from pin 3 to 4 and pin 5 to 6. The PT Secondary wires are connected to pins 3 and 5. The bands of the diodes are on pins 4 and 6. I'm pretty sure that is how things should be wired.
Any thoughts on what would cause the voltages to be so low?
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Here's a picture of the rectifier socket.
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If there are no filter caps connected to the rectifier, the dc voltages will be way low.
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If there are no filter caps connected to the rectifier, the dc voltages will be way low.
Here's another view where you can see the power supply wiring.
There are two red wires connected to Pin 8 of the rectifier; one from the OT and one that goes to the first filter cap.
The 10K voltage dropping resistor is soldered directly to the terminals of the dual 32uF filter cap.
A red wire goes from the second cap to Pin 6 of V5 and V4. I have Screen resistors going from Pin 6 to Pin 4.
A red wire goes from Pin 6 on V5 to the second 10K voltage dropping resistor where you can see the 16uF filter cap is connected (I don't show ground, but its there).
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Remove the power tubes and leave them out until you resolve the B+ issue. Measure resistance from rectifier socket pin 8 to chassis. Should be high resistance.
Are you using a light bulb limiter? If so, remove it. Any better?
Change rectifier tube. Any better?
Use a couple gator clip leads to short the "backup diodes". Any better?
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Remove the power tubes and leave them out until you resolve the B+ issue. Measure resistance from rectifier socket pin 8 to chassis. Should be high resistance.
When I measure Pin 8 of rectifier to ground I get 466K but it continues to climb slowly. Perhaps that's a clue.
I checked the voltages at Pin8/C1, C2 and C3 without the power tubes and measured 430, 420 and 410VDC respectively.
Are you using a light bulb limiter? If so, remove it. Any better?
Change rectifier tube. Any better?
Use a couple gator clip leads to short the "backup diodes". Any better?
No light bulb limiter... I have one but could not find the incandescent bulbs after we moved and my local stores only have LEDs. I only have one 5V4G rectifier... I could swap in a 5AR4, but not sure that is the issue after seeing the higher voltages when the Power Tubes are removed. I didn't try bypassing the diodes for the same reason.
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Power supply seems OK. Put the tubes back in. Measure voltage on pin 5 and 8 of each power tube. What have you?
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Power supply seems OK. Put the tubes back in. Measure voltage on pin 5 and 8 of each power tube. What have you?
I put all the tubes back in, not just the power tubes.
V4, Pin 5 = 0.007 VDC* (Control Grid)
V4, Pin 8 = 13.7 VDC (Cathode)
V5, Pin 5 = 0.008 VDC*
V5, Pin 8 = 13.7 VDC
*Voltages on Pin 5 fluctuated a bit. Measured on my Fluke 77V.
While I was at it I measured additional points:
C3 = 182VDC.
Two 270R resistors come together in a upside down V = 182VDC (it should be because there is an under-board jumper between C3 and those resistors).
V3, Pin 1 = 108VDC (this is connected to the other side of one of the 270R resistors) (plate)
V3, Pin 6 = 107VDC (this is connected to the other side of one of the 270R resistors) (plate)
After 0.047uF cap = 0.008VDC (connected to V5, pin 5 so makes sense)
After 0.022uF cap = 0.007VDC (connected to V4, pin 5 so makes sense)
I have included a photo of my wiring.
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Put your 5AR4 in and short out the backup diodes.
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Put your 5AR4 in and short out the backup diodes.
I think we are making progress. I swapped in a 5AR4 and measured (with diodes and with diodes bypassed - didn't make a difference).
| Point | | Expected | | Measured | | Measured w/5AR4 |
| PT Secondaries | | 300 VAC | | 316 VAC | | 316 VAC |
| Rectifier Pin 4 and 6 | | ? | | 146 VDC | | 311 VDC |
| C1 | | 388 VDC | | 197 VDC | | 391 VDC |
| C2 | | 308 VDC | | 175 VDC | | 337 VDC |
| C3 | | 278 VDC | | 160 VDC | | 309 VDC |
| V5 Pin3 | | 385 VDC | | 192 VDC | | 387 VDC |
| V5 Pin5 | | ? | | .007 VDC | | .016 VDC |
| V5 Pin8 | | ? | | 13.7 VDC | | 25.56 VDC |
| V4 Pin3 | | 385 VDC | | 192 VDC | | 386 VDC |
| V4 Pin5 | | ? | | .008 VDC | | .022 VDC |
| V4 Pin8 | | ? | | 13.7 VDC | | 25.56 VDC |
| V3 Pin1 | | ? | | 108 VDC | | 190.6 VDC |
| V3 Pin6 | | ? | | 107 VDC | | 189.3 VDC |
The voltages with the 5AR4 are in the ballpark of what I expected although higher. But, that is to be expected as well since the 5AR4 does not drop as much voltage as the 5V4G
Do you think I have a bad 5V4G tube or could this be caused by using the 32uF filter caps? The datasheet says additional plate impedance may be necessary for 40uF filter caps.
I went ahead and plugged in a speaker/guitar. Seems to sound ok.
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bad tube.
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bad tube.
Ugh! :cussing: :sad2: :BangHead:
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I ordered a 5V4G (two actually) so I will wait until they arrive for an "official" tone reports but I brought the amp up to play with some real speaker cabinets. My bench speaker is a 5-inch 30 watt speaker that is good for proving the amp makes sound, but not much else.
It's pretty quiet. A slight hiss when the volume knobs are turned past noon, but nothing out of the ordinary. No hum or buzz. The channels sound different from each other which is expected; One is cathode biased, the other is grid leak biased. There is not a lot of clean - which seems to be a thing with my amps. :-D I suspect some of that has to do with the higher voltages from using the 5AR4 instead of the 5V4G.
I think I wired the tone knob backwards. Not sure if that is how Supros are ordinarily wired or if I messed up. Regardless, when I turn the knob clockwise it gets darker and loses the highs. I will probably re-wire that so it is more intuitive.
The Tremolo does not work at all. Doesn't matter if I use a foot switch or if I use the push/pull switch on the speed knob so that will need some debugging. I will probably wait until the new rectifier arrives to work on that.
The one thing that is sort of disappointing... it doesn't really sound like the Jimmy Page amp. At least right now. I'm going to wait until I get the proper rectifier to mismatch the load (16 ohm speaker in a 4 ohm output). See how that goes. That said, the amp sounds pretty darn good right now even if I don't sound like Zepplin I.
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which seems to be a thing with my amps.
Mine started that way also, the biggest reason in mine, too much gain, slamming the "next" stage
the other was cap values too big, early in the stages
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No need to wait for a new rectifier tube before troubleshooting the tremolo.
You did use two insulating bushings on the footswitch jack, right? And you verified that the sleeve lug does not read zero ohms to chassis, right? Disregard my questions about the footswitch jack since you are using a "Cliff" style jack.
I suggest you eliminate the switch and footswitch jack first. This can easily be done by disconnecting the black wire from the FS jack and disconnecting the violet wire from the speed pot switch (referring to your latest layout). Now connect the black and violet wires together and leave like this until the tremolo is working. If the tremolo is already working with the switches bypassed then you only need to examine that wiring. If not, then double check the wiring of the tremolo tube and circuit, paying close attention to the fact that your layout shows using pins 6, 7, 8 for the tube but your schematic shows using pins 1, 2, 3. I've looked closely at your layout in the area that deals with tremolo and see no problems, so if your amp is really wired IAW your layout, the tremolo should work. Post the voltage readings for pins 6, 7, and 8.
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Not sure what I was thinking but I totally botched the foot switch. I think it’s because I made the faceplate match the original control layout without double checking my component layout. So I had to swap the pots and then I rotated the jack but when I wired I didn’t look at what the lugs do, just followed the wires.
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So, is the tremolo working now?
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I just fixed the wiring and no luck.
I’m using switchcraft jacks now because I wanted them all to match and could not did a cliff equivalent of the 13A.
To answer your question, sleeve to ground is 67K with the pot switch in, 98K with it out.
With the pot pushed in
V2, 6 = 229
V2, 7 = 0
V2, 8 = 2.7
With the pot pulled out
V2, 6 = 72
V2, 7 = -0.484 (negative???)
V2, 8 = 0.1
I will have to try connecting black and violet wires later on. I have other commitments now. Do I need to disconnect both wires or could I only disconnect the black wire and then use clips to connect it to the violet wire in place? The way I installed the ground bus makes it a little tight.
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I’m using switchcraft jacks now because I wanted them all to match and could not did a cliff equivalent of the 13A.
So, did you use two isolating bushings on the F/S jack?
To answer your question, sleeve to ground is 67K with the pot switch in, 98K with it out.
Sleeve to ground should be 3.9K and should not change when you mess with the pot switch. This 3.9K resistance is provided by the 3.9K resistor on V1 pin 8.
I will have to try connecting black and violet wires later on. I have other commitments now. Do I need to disconnect both wires or could I only disconnect the black wire and then use clips to connect it to the violet wire in place? The way I installed the ground bus makes it a little tight.
I was very clear about those wires.
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So, did you use two isolating bushings on the F/S jack?
Yes. When I use the continuity checker on my meter I get no sound.
Sleeve to ground should be 3.9K and should not change when you mess with the pot switch. This 3.9K resistance is provided by the 3.9K resistor on V1 pin 8.
I disconnected the black wire and it measures 3.9K as it should. Maybe that's a a clue.
I connected the black and violet wires (its actually yellow in real life, but we can keep calling it violet for clarity)... still no tremolo. You only asked for the voltages on V2, Pins 6, 7 and 8 but I have included all the voltages for V1 and V2:
V1, 1 = 92V
V1, 2 = -0.833V
V1, 3 = 0V
V1, 6 = 193V
V1, 7 = 0V
V1, 8 = 2.27V
V2, 1 = 48V
V2, 2 = -0.15V
V2, 3 = 0.513V
V2, 6 = 201V
V2, 7 = 0V
V2, 8 = 2.27V
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Sleeve to ground should be 3.9K and should not change when you mess with the pot switch. This 3.9K resistance is provided by the 3.9K resistor on V1 pin 8.
I disconnected the black wire and it measures 3.9K as it should. Maybe that's a a clue.
The black wire should read 3.9K because the other end is connected to a 3.9K cathode resistor. So, it's incredible to have "sleeve to ground is 67K with the pot switch in, 98K with it out". What resistance do you have between the sleeve and chassis while the black and violet wires are conned together?
V2, 6 = 201V
Is this voltage steady or is it bouncing around (would look erratic on a DMM)? And what is the voltage at the Node C filter cap?
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With the black and violet wires connected I did not get any reading between sleeve and chassis. Just to be sure I removed the jack from the chassis so it is floating in air. There is no way it can be making a ground connection now. Still no Tremolo.
The voltage on V2,6 is steady although 200V this time. I took voltages at various points throughout the circuit and annotated the layout in the attachment.
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With the black and violet wires connected I did not get any reading between sleeve and chassis.
And that's what it should be. So, your statement "sleeve to ground is 67K with the pot switch in, 98K with it out" is still a mystery. That's OK. Reinstall the jack and connect the black and violet wires back as they should be and lets start over.
The steady plate voltage means the trem oscillator is not oscillating. Set the INT pot to mid rotation and be sure nothing is plugged into the F/S jack. With amp off, measure resistance from pin 8 to ground. It should be 18.9K with the pot switch off and 3.9K with the pot switch on.
Turn the amp on and measure the voltage on V2 pins 6, 7, and 8 with the pot switch off. Now turn the pot switch on and repeat voltage measurements for V2 pins 6, 7, and 8. Also verify that V2 pin 6 voltages changes as you turn the INT pot.
Also post some hi-rez pics of the guts. I'm mostly interested in seeing V2 and the portion of the board with the tremolo circuit and connecting wires. It'll be good if I can see board jumpers too.
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That's OK. Reinstall the jack and connect the black and violet wires back as they should be and lets start over.
I added extra insulation (clear enamel) to the jack and the chassis just in case. Now when I reinstall the jack the there is a 3.9K no matter how things are switched. That seems to be sorted out, but still no Tremolo.
The steady plate voltage means the trem oscillator is not oscillating. Set the INT pot to mid rotation and be sure nothing is plugged into the F/S jack. With amp off, measure resistance from pin 8 to ground. It should be 18.9K with the pot switch off and 3.9K with the pot switch on.
18.86K and 3.9K - close enough.
Turn the amp on and measure the voltage on V2 pins 6, 7, and 8 with the pot switch off. Now turn the pot switch on and repeat voltage measurements for V2 pins 6, 7, and 8. Also verify that V2 pin 6 voltages changes as you turn the INT pot.
V2,6 Off = 265
V2,7 Off = 0
V2,8 Off = 3.39
V2,6 On = 204
V2,7 On = 0
V2,8 On = 2.28
Also post some hi-rez pics of the guts. I'm mostly interested in seeing V2 and the portion of the board with the tremolo circuit and connecting wires. It'll be good if I can see board jumpers too.
Oops... wrong images. hard to tell from the thumbnails on my computer.
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Here is the pots and jack - yeah should have routed that black wire better. :-D
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V2 Socket... nothing to see here.
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turret board components.
The red jumpers are on top because I accidentally soldered the wire in the wrong turret when I was first running the jumpers. Because the wire was too short, I had to cut a new wire but then the solder that was left in the turret was keeping me from being able to install the jumper under the board again so I just ran them on top.
Red wire from the intensity pot is connected under the board to the turret where the other red wires are connected.
Blue wire from intensity pot is connected under the board where the 270K and 100K resistors come together.
All other jumpers are on top of the board.
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I really want to see one pic that shows the board and V2 socket together.
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Here you go
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I was just reviewing Valve Wizard's write-up of The Phase-Shift Oscillator For Tremolo (http://www.valvewizard.co.uk/trem1.html).
It seems like all the parts are there but there are a few differences not described in his page:
1. The intensity pot... I'm not quite sure how to calculate Ra. Would it be 270K or would it be 270K + (500K || 150K).
2. He uses the same values for all the caps while the 1690T has different caps. I can't imagine that is the issue.
3. The resistors for R1, R2, R3 are all quite different that what he suggests.
4. This is the biggest difference... he shows the foot switch being between C2 and C3 while the 1690T has it off the cathode. I haven't compared to other amp schematics yet. Could the schematic be wrong?
I keep re-tracing the circuit and I'm not seeing any other issues. By the way, I swapped V2 for a different tube. Just in case that was the issue. Nope... same behavior. No Tremolo.
I also had the sleeve and shunt connected which the schematic doesn’t so I removed that. Everything is connected to the sleeve only now. Still no tremolo.
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I also had the sleeve and shunt connected which the schematic doesn’t so I removed that. Everything is connected to the sleeve only now. Still no tremolo.
The schematic clearly shows the sleeve and shunt connected. The circuit cannot possibly work with that connection removed.
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The schematic clearly shows the sleeve and shunt connected. The circuit cannot possibly work with that connection removed.
You are right (of course). I got myself confused trying to find something I missed. I put it back. No difference.
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Is the amp back to your original wiring? If so, I have a couple more things to try...
First, turn on the pot switch and be sure nothing is plugged into the F/S jack. Now measure resistance from V1 pin 8 to V2 pin 8. Should be zero ohms. Next measure resistance from V2 pin 8 to chassis. Should be 3.9K. Are both readings correct?
Now put a temporary jumper across the two INTENSITY pot lugs ( the ones with the wires connected). This takes the INT pot out of the circuit and should result in maximum tremolo intensity. Turn the amp on. Do you have tremolo now?
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Is the amp back to your original wiring? If so, I have a couple more things to try...
Amp is back to original wiring. Shunt and Ground are wired together. When nothing is plugged in, Shut, Ground and Tip are all connected. If I plug in a cord (not a footswitch) then the Tip is no longer connected to Ground.
First, turn on the pot switch and be sure nothing is plugged into the F/S jack. Now measure resistance from V1 pin 8 to V2 pin 8. Should be zero ohms. Next measure resistance from V2 pin 8 to chassis. Should be 3.9K. Are both readings correct?
Pretty much... when I connect the leads of my DMM directly to each other I am getting 0.02 ohms. When I am measuring pin 8 to pin 8 I'm getting 0.05 ohms. This is the same value I'm getting at the switch.
Now put a temporary jumper across the two INTENSITY pot lugs ( the ones with the wires connected). This takes the INT pot out of the circuit and should result in maximum tremolo intensity. Turn the amp on. Do you have tremolo now?
Nope.
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I got a NOS 5V4G today and put it in the amp. Tremolo still doesn't work but I didn't expect that to make a difference. Here are the latest voltages:
C1 = 382V
C2 = 330V
C3 = 302V
V1, 1 = 90V
V1, 3 = 0V
This is where things get interesting...
V1, 6 = 90V initially seems to fluctuate increasing to 140V and keeps climbing.
V1, 8 = 1.8V when the pot switch is off (pushed in).
V1, 8 = 1.9V when the pot switch is on (pulled out), but changed over time to 2.3V. Related to V1,6 behavior???
V2, 1 = 50V
V2, 3 = 0.5V
V2, 6 = 254V
V2, 8 = 3.3V with switch off, intensity pot on 0
V2, 8 = 3.2V with switch off, intensity pot on 10
V2, 8 = same values as V1,8 with switch on.
Power tubes... I don't think there is anything important here.
V4, 3 = 377V
V4, 4 = 329V
V4, 8 = 24.9V
V5, 3 = 378V
V5, 4 = 329V
V5, 8 = 24.9V
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Double check your wiring and component values in the trem circuit.
We gotta get that trem tube oscillating. Here's a test to do that will make it easy to tell when the tube is oscillating. Disconnect the yellow wire from the pot switch. (I'm looking at the pic, not the layout.) Connect the anode of a red LED to the end of that yellow wire and connect the cathode of that LED to the ground buss. Now we have a visual indicator. The goal is to make that LED flash. There will be no tremolo signal sent to V1 while the LED is connected. It's possible that the LED will be flashing without doing anything else.
If the LED is not flashing then change the three caps in the trem circuit.
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Double check your wiring and component values in the trem circuit.
We gotta get that trem tube oscillating. Here's a test to do that will make it easy to tell when the tube is oscillating. Disconnect the yellow wire from the pot switch. (I'm looking at the pic, not the layout.) Connect the anode of a red LED to the end of that yellow wire and connect the cathode of that LED to the ground buss. Now we have a visual indicator. The goal is to make that LED flash. There will be no tremolo signal sent to V1 while the LED is connected. It's possible that the LED will be flashing without doing anything else.
If the LED is not flashing then change the three caps in the trem circuit.
Not sure I have a Red LED handy. This is when I really miss Radio Shack. I wish places would ship a part like that in an envelope instead of charging $5 to send in a giant box. I can buy one from Mouser and pay $7.99 in shipping or I can get a box of 100 LEDs in 5 colors sent to my home with free shipping for $4.99.
Any thoughts or concerns about the V1, Pin 6 voltage changing like that? Pin 1 voltage doesn't seem to change. I will admit I haven't left the amp on for more than 10 minutes at a time. Could this just be to the tubes warming up?
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You don't really need the LED. Just leave everything connected normally. Set the speed pot to midrange. Connect your DMM to V2-6 to monitor plate voltage. If the voltage is steady, the oscillator is not working. If the voltage is changing erratically, the oscillator is working. If your DMM has a bar graph (like my Fluke 87V) you can see the plate voltage actually changing smoothly at the speed of the oscillator.
Change those 3 caps if the oscillator is not working.
I'm not concerned about the plate voltages on V1 at this time. But tell me, how does the amp sound, both channels?
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We used to be able to pull LEDs out of old computers and appliances.
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But tell me, how does the amp sound, both channels?
Power was out here all day so I didn’t get a chance to check voltages. LEDs will here tomorrow so I will try that.
I brought the amp up and plugged it in to a proper cabinet. The tremolo channel is grindy but articulate. Plugging into the treble jack is almost like a treble booster.
The normal channel has a bunch of bass but it’s tight with no tubby or flubbiness. It’s cleaner and bigger but still has some grit. Although, my guitar has some hot P90s that makes every amp have some grit so I hoots try it with my Tele tomorrow.
So how does it sound… It sounds like the gods of rock have decided to sprinkle fairy dust into the amp. it’s really awesome. Unlike anything else I have.
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With the speed mid range and intensity pot all the way clockwise, The voltage on V2, six is steady at 199 V. When I turn the tremolo off the voltage increases to 256 V.
If I change the intensity pot to be all the way counterclockwise, then the voltage increases to 215 V. If I turn off the tremolo than the voltage is 267 V.
The voltage is steady no matter what I do.
Should I replace the capacitor is one at a time or do all of them at once?
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I would do all three. Don't throw them away.
Are you absolutely sure it's wired correctly? I can't tell from looking at the pics because you're feeding the off board wires under the board. Use your ohm meter to verify all wires go to the correct place. The layout agrees with the schematic so just make sure the wires are connected correctly. Do this before changing the caps.
I'm thinking the INT pot may be connected backwards but that's not a biggie. Just set it mid way. And, remember, when the oscillator is working the plate voltage will be fluctuating.
What DMM do you have?
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I would do all three. Don't throw them away.
Hopefully, I have spares in my drawer so I don't have to wait for new ones to arrive.
Are you absolutely sure it's wired correctly? I can't tell from looking at the pics because you're feeding the off board wires under the board. Use your ohm meter to verify all wires go to the correct place. The layout agrees with the schematic so just make sure the wires are connected correctly. Do this before changing the caps.
I verified using my continuity tester before connecting them. I can try to lift up the board and get my phone underneath to take a picture.
I'm thinking the INT pot may be connected backwards but that's not a biggie. Just set it mid way. And, remember, when the oscillator is working the plate voltage will be fluctuating.
The schematic says speed pot works backward. Maybe I wired the intensity pot backward too.
What DMM do you have?
Fluke 77V - no graphing.
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No need to take a pic. I believe you.
Your DMM does have a bar graph and it will 'pulse' at the rate of your tremolo speed.
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I don't know if you've seen this, very good way to double check a builds wiring;
https://el34world.com/Forum/index.php?topic=17701.0
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Fluke 77V - no graphing.
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Sluckey, do you remember Phsyconoodler couldn't get the LFO in a 18w Marshall he built?
I suggested he try a 470K for the LFO plate R just to see if it would oscillate, and the 470K kicked the LFO in gear. He ended up going down to ~300K. Stock was a 220K LFO plate R. He tried many 12AX7's, nothing worked until he upped the LFO plate R's value.
dbishopbliss, try a 470K R for the LFO plate and see if it kicks it into oscillating. If it does, then keep lowering that plat R's value until it stops. Then up the value back up a little till it oscillates again. Just use gator clip cables.
Here's the post of what he finished with;
https://el34world.com/Forum/index.php?topic=16282.msg160098#msg160098 (https://el34world.com/Forum/index.php?topic=16282.msg160098#msg160098)
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Ok, Phsyconoodler's 18w Marshall, stock the LFO plate R is 100K. He tried a 220K with a red cathode LED, that combo didn't work. He then went up to a 470K LFO plate R and a red LED for the LFO cathode. That combo kicked the LFO in gear.
But then it was thumping too much so he ended up taking out the LED, going back to an R with a bypass cap for the cathode and lowering the value of the LFO plate R down to 330K. That combo worked well for him.
Looking at the schematic, your Supro's LFO has either a 220K or a 270K?
Try going up to 470K for the LFO plate R. (You might need to also use an red LED with that 470K, just to get it going.) That might kick it in. Any LFO wont oscillate until there's enough gain.
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I built a 5G9 and couldn't get the LFO to oscillate. :BangHead: :cussing:
Went over the wiring and parts values many times, all was good. :help: :think1:
Finally changed the LFO 9 pin socket, ceramic with cinch pins. That fixed it. Must have been a bad socket.
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If you look at the schematic you'll see the plate is connected to a 270K in series with a 150K that has a 500K pot across it. That means the plate resistance can be varied from 270K up to 385K.
But yes, if the oscillator is on the edge of oscillating, increasing the gain by increasing the plate load is a possibility. Worth a try. There's no need for a LED in this circuit. I suggested using a LED only for a visual indicator while troubleshooting. But his meter has the capability to show the plate voltage pulsing in time with the trem speed.
I also suggest increasing the B+ by moving from node C to node B.
But all this is moving toward modifying the circuit. I'd really like to see it operate with the provided schematic, but who knows if it is even correct?
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Your DMM does have a bar graph and it will 'pulse' at the rate of your tremolo speed.
So that is what that is... I was thinking something like this...
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If you look at the schematic you'll see the plate is connected to a 270K in series with a 150K that has a 500K pot across it. That means the plate resistance can be varied from 270K up to 385K.
I'm going to lift one end of the 150K resistor. That will allow me to go go up to a 770K load and will be easy to adjust without soldering... plus I don't have a box of resistors to swap in.
But yes, if the oscillator is on the edge of oscillating, increasing the gain by increasing the plate load is a possibility. Worth a try. There's no need for a LED in this circuit. I suggested using a LED only for a visual indicator while troubleshooting. But his meter has the capability to show the plate voltage pulsing in time with the trem speed.
Now I know what to look at.
I also suggest increasing the B+ by moving from node C to node B.
I will try that after removing the 150K resistor.
But all this is moving toward modifying the circuit. I'd really like to see it operate with the provided schematic, but who knows if it is even correct?
And that is a good question. The schematic was redrawn by "Dirty Girl Amps" in 2013. That doesn't mean anyone has actually built an amp using it. I will try reaching out to Leon C on another forum. I know he built one, but I don't know what schematic he used.
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I will try reaching out to Leon C on another forum.
Good idea. Meanwhile try your good idea to remove the 150K.
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Well this is annoying... BEFORE CHANGING ANYTHING, I decided to check out the graphing feature that I didn't know I had. I connected the meter to ground and V2, Pin 6. This time the voltage was varying quite a bit. I didn't even need a graph to see it.
So I plugged in a guitar to see if I could hear anything. Nope... no tremolo. Just for grins I decided to connect the meter to V2, Pin 8. Suddenly, I could hear tremolo!!! It doesn't do the "whump whump whump" or go as slow as the Vox, but there is definite audible tremolo. As expected, the voltage on the cathode is varying too. So I disconnected the meter and the tremolo is still there. I poked around with a chopstick but I get no crackling or sound going in and out. I'm going to reheat the joints on the tube socket pins anyway and test again after the tubes have cooled.
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Update... it seems to come and go. Sometimes I can hear the click click click of the tremolo but there is no tremolo. Other times it comes back in. I did notice some pots here and there as I was playing with the Intensity pot. But those seemed to have gone away.
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Remove the 150K
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I will try removing the 150K resistor later today... in the meantime, here is the reply I got from Leon C asking about the schematic he used.
The tremolo circuit in mine is identical to the one in Eric's schematic. Mine has the 6.8k rather than the 15k resistor between the oscillator cathode and on/off jack ring. The previous owner added a 3.3uF cap in parallel with the .001 that goes to ground. I haven't removed it as it works fine as is.
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Ugh... things are getting worse. I swapped in a different set of power tubes just to see if I could hear a difference. Turned it on, hummmm. Then POP! Then smoke!!!! Tubes look ok. Do fuses make smoke? I haven't opened it up yet.
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What kind of tubes? Fuses don't usually smoke, at least enough to see it.
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Just for grins I decided to connect the meter to V2, Pin 8. Suddenly,
that sorta thing, makes smoke. maybe time for a new socket and associated soldered parts?
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Just for grins I decided to connect the meter to V2, Pin 8. Suddenly,
that sorta thing, makes smoke. maybe time for a new socket and associated soldered parts?
No smoke when I did that. Things started working.
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What kind of tubes? Fuses don't usually smoke, at least enough to see it.
Initially I was using a pair of Sovtek 5881/6L6WGC. I replaced those with a pair of Svetlana 6L6WGC. I think they were originally in the Bandmaster i gave you. Actually, they were the first pair of tubes I tried when I fired up the amp for the first time. I think there was a big hum then too. Time to go open it up see what I can find.
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Looks like I lost the cathode bypass cap.
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I just cleaned up the guts and replaced the cap. Lucky for me I had a 22uF 50V cap, which is close enough to 25uF. I installed the working tubes and everything but the tremolo seems to be working again.
I lifted a leg of the 150K resistor. Still no tremolo. The voltage is changing but there is no change in the sound (no whump whump whump). I've included a link to a video so you can see and hear what is happening.
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Frequency seems very fast. Put your meter in AC mode. Measure the signal voltage. Switch to freq. mode and check the frequency. Verify that the frequency changes as you turn the speed pot. Refer to manual if you don't know how to check frequency...
https://dam-assets.fluke.com/s3fs-public/77iv____umeng0100.pdf
Post the voltage and frequency (min and max) numbers.
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Frequency seems very fast. Put your meter in AC mode. Measure the signal voltage. Switch to freq. mode and check the frequency. Verify that the frequency changes as you turn the speed pot. Refer to manual if you don't know how to check frequency...
https://dam-assets.fluke.com/s3fs-public/77iv____umeng0100.pdf (https://dam-assets.fluke.com/s3fs-public/77iv____umeng0100.pdf)
Post the voltage and frequency (min and max) numbers.
Min: 67.5
Max: 81.2
Frequency changes as I turn the speed pot.
Here's another video for your viewing pleasure.
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I want the frequency min max numbers, ie, turn the speed pot max CCW and note frequency. Then turn the speed pot max CW and measure the frequency. What is the slowest trem freq? What is the fastest trem freq?
67VAC trem signal should be plenty. Once again, disconnect the violet wire from the pot switch and disconnect the black wire from the FS jack. Connect those two wires together. This totally bypasses the switching circuit and ties the oscillator cathode directly to V1-8.
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I want the frequency min max numbers, ie, turn the speed pot max CCW and note frequency. Then turn the speed pot max CW and measure the frequency. What is the slowest trem freq? What is the fastest trem freq?
67VAC trem signal should be plenty. Once again, disconnect the violet wire from the pot switch and disconnect the black wire from the FS jack. Connect those two wires together. This totally bypasses the switching circuit and ties the oscillator cathode directly to V1-8.
With the intensity pot half way and the switch still in the circuit:
Min frequency is 5.07
Max frequency is 8.62
With the switch out of the circuit:
Min is 4.88
Max is 8.12
It seems like I can hear a very very very faint tremolo.
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It seems like I can hear a very very very faint tremolo.
I'm thinking that was wishful thinking and really just the wobble of strings out of tune.
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Did you do this?
Once again, disconnect the violet wire from the pot switch and disconnect the black wire from the FS jack. Connect those two wires together. This totally bypasses the switching circuit and ties the oscillator cathode directly to V1-8.
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Did you do this?Once again, disconnect the violet wire from the pot switch and disconnect the black wire from the FS jack. Connect those two wires together. This totally bypasses the switching circuit and ties the oscillator cathode directly to V1-8.
Yes. That is what I meant by saying, “with the switch out of the circuit “
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What is the voltage of the trem signal? Meter set to AC and connected to plate (V2-6).
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What is the voltage of the trem signal? Meter set to AC and connected to plate (V2-6).
Voltage oscillates between 67 and 84 VAC.
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OK, there's nothing wrong with the oscillator. Try this... Connect a gator clip test lead jumper directly from V1 pin 8 to V2 pin 8. Any joy?
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Any joy?
No joy. :sad2:
I measured the DC voltage on the cathodes and it was between 1.8 and 2.1 V (something like that didn't write it down).
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I'm reading Valve Wizard Phase-Shift Oscillator for Tremolo page again...
It says, The effect requires a low frequency oscillator (LFO), which produces a wave (usually a sine wave) of around 0.5 to 10Hz- the "trem' signal". This is then mixed with the audio signal in some way, so that its amplitude (volume) increases and decreases in sympathy with the trem' signal.
The diagram that accompanies the description shows a sine wave signal coming from the anode of the tube. If I am understanding things correctly, the Supro is changing the voltage on the cathode of V1 but it doesn't appear that anything is being "mixed with the audio signal."
Is there a missing connection in the circuit or should the changing voltage be audible?
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I probably missed it;
when you plug your guitar into the Trem input jacks does the guitar sound get through? all the Vol/Tone pots work?, just no trem?
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Is there a missing connection in the circuit or should the changing voltage be audible?
That's why I had you directly connect V1 pin 8 to V2 pin 8. You cannot hear the actual tremolo sine wave. It's too low frequency. But you can hear the effect it has on varying the gain of V1B. It's called amplitude modulation and it sounds exactly like someone quickly turning the volume pot up and down while you are playing.
I've looked at every Supro, Valco, and Gretsch schematic in Hoffman's library. There are only two other cheap models that use a one triode tremolo circuit similar to yours. Everything else has a cathode follower on the output of the oscillator. That suggests to me that your one triode oscillator circuit is inferior and not reliable. Probably why they switched to the two triode circuits.
Whatever, I'm out of ideas. Jimmy never used tremolo anyway! :l2:
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I probably missed it;
when you plug your guitar into the Trem input jacks does the guitar sound get through? all the Vol/Tone pots work?, just no trem?
Yep. Everything else works and sound good.
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has the bypass cap on V1A been removed from the circuit? If not, lift one leg n test
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has the bypass cap on V1A been removed from the circuit? If not, lift one leg n test
I’m confused… V1A is not in the tremolo channel and doesn’t have a bypass cap… at least on the cathode.
Are you referring to the 35uf cap on V1B cathode? Haven’t tried that.
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I'm following with this schematic, so;
Supro_1690t.pdf (el34world.com) (https://el34world.com/charts/Schematics/files/Supro/Supro_1690t.pdf)
IF the mix point for dry signal and trem is a cathode and has a bypass cap, (like the linked schematic), lift it n test.
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I'm following with this schematic, so;
Supro_1690t.pdf (el34world.com) (https://el34world.com/charts/Schematics/files/Supro/Supro_1690t.pdf)
IF the mix point for dry signal and trem is a cathode and has a bypass cap, (like the linked schematic), lift it n test.
Yep, that's the one. I will give it a try tomorrow. Today is a busy day.
Another thing I need to try is swapping the 15K resistor off of the foot switch for a 6.8K. That is what Leon C has in his amp and was marked as a variant on that schematic. Although, I bypassed it entirely with some of Sluckey's suggestions so I can't imagine that would make a difference. Leon C also said he has a 3.3uF cap in parallel with the .001 cap. I tried clipping in a 2.2uF because that is the closest I have but it didn't make a difference.
I got some time to play at full volume for about 2 hours yesterday. The amp sounds great and there is a slight barely noticeable change in tone when the tremolo is engaged. And there is a loud pop when the switch is engaged. So something is happening. Just no whump whump whump.
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At this point, I'd change the tube socket and the wires that go to it.
We have seen more than a couple times, someone could not fix the amp and out of desperation they changed the tube socket and that was it.
You have nothing to lose at this point.
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Apologies for jumping back into this topic after a couple years, but my own 1696t build is nearing completion and I just wanted to share a couple of items I encountered, dbishopbliss.
1.) My tremolo was annoyingly subtle. The end result, following sLuckey's comments, was to remove the 1K cathode resistor on V3b and replace it with an LED. Steve does this in his Supro Model S-6424 (12/10/2020). Like magic, the tremolo is now loud and proud! Highly recommended.
2.) On first power up, I also blew my 33uF 50V cathode bypass cap in the power section. I ended up replacing it with a 160V cap for extra insurance. Glad to hear your 50V cap is doing the trick.