Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on August 01, 2018, 04:22:25 pm
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Well, today I received a giant pile of goodies from the Hoffman store and am ready to start next build. Each one is more fun than the last!
In the schematic below, I have removed one of the two channels I had in the original schematic/build, which people here helped me with tremendously a few months ago. I've removed it because the person who will be playing it won't need two and wants a simpler control panel. So as a result this will give me one unused triode on the 12AY7 preamp tube.
My question is: If you were building this amp, what would you use that unused triode for? Looking to have some fun and come up with something cool for it. Thanks! I'm open to anything; one thing I hope can be pulled off (but I don't know) is to find a single triode tremolo circuit that will work well in a cathode bias amp. Does one exist?
About the amp: It's basically a 5E3 with a Princeton reverb circuit stuck in the middle of it. When I first built it I didn't account for the decrease in gain that the reverb circuit would create, so unlike the standard 5E3 it has an extra gain stage.
There are a few things not on the schematic that I'm going to do (voltage doubler for reverb to bring down the level a bit, etc.) but nothing that will/should have to change due to the dropping of one channel.
Thanks!
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Here's a trem circuit that will work well with cathode biased 6V6s...
http://sluckeyamps.com/RCA/Ampeg_J12B.pdf
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Thanks Sluckey. This looks great! A few questions:
1. Yours calls for a 6SL7, I'll be using half a 12AY7. Will I need to account for the differences in gain?
2. Also, the LED shown on your schematic. Is that the bug you see a lot on Fender amps? Have never built a tremolo with one of those bugs before so it's an unknown to me.
3. Finally, will lead dress be an issue? I'll probably just insert all the tremolo stuff where it makes sense, right next to the preamp area of the circuit board I'll build. Or should I move it?
4. As for the slow/fast switch, I'll probably just hard-wire one or the other (more likely the slow half of it) to cut down on clutter on the instrument panel.
Thanks for the great suggestion.
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Tremolo oscillators like a high gain triode. That 12AY7 may not work reliably. Just try it and see. You can always plug a 12AX7 in the socket.
The LED is just a simple red or yellow LED. Nothing fancy but you have to observe polarity. The LED may enable the 12AY7 to work. Try and see.
Lead dress is not an issue for the trem circuit.
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Not an issue on the 12AY as I have a feeling he's going to want the higher gain of the AX anyway. This is the perfect solution for that half a tube. Thanks again.
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How's this look for an LED? Correct?
So, I take it that as the intensity of the tremolo is turned up, the LED will grow brighter? And/or will it pulsate at different rates depending on speed? If so that would be very cool!
https://www.alliedelec.com/rs-pro-205577/70825080/?mkwid=snFxSRnoe&pcrid=30980760979&gclid=CjwKCAjw14rbBRB3EiwAKeoG_5vI_bOeiSVbOOe233GfnNzQfeRKQm-OmGIW3_DaN0Puy7Ay4zkoKRoCFU4QAvD_BwE
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That LED is fine. I have a similar LED bezel on the front panel of my Revibe and my Hampeg.
The LED will pulse at the speed of the tremolo. The brightness is constant, ie, it doesn't vary with the intensity pot. The LED will be off when the footswitch turns the tremolo off.
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Great. That sounds fantastic. Now to find one where the shipping cost isn't 20X the price of the LED :BangHead:
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5mm red LEDs and bezels are dirt cheap on eBay. That's where mine come from. Also pretty cheap at Radio Shack if you still have a nearby store.
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Unfortunately all RS stores are gone locally.
I'm looking at this one, but there's no clue in the description about the ma rating. Link to data sheet does not work.
https://www.ebay.com/itm/1-Dialight-607-1112-110F-607-Series-2V-Red-Protruding-Panel-Mount-LED/201900452065?epid=1417968188&hash=item2f023468e1%3Ag%3AvXkAAOSw7U5Y-mVT%3Asc%3AUSPSFirstClass%2102809%21US%21-1&LH_PrefLoc=1&_nkw=2v+LED+panel&_sacat=0&_from=R40&rt=nc&LH_TitleDesc=0%7C0
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Something like this...
https://www.ebay.com/itm/150pcs-3mm-5mm-LED-Light-White-Red-Green-Yellow-Assorted-Emitting-Diode-DIY-Set/391368927437?epid=2284726737&hash=item5b1f67e8cd%3Ag%3AU5sAAOSwCypWpsoN&LH_PrefLoc=1&_sacat=0&_nkw=5mm+red+led&_from=R40&rt=nc
and this...
https://www.ebay.com/itm/20-x-5mm-Bezel-LED-Holder-Chrome-Metal-USA-Seller-Free-Shipping/232807780503?hash=item36346cd097%3Ag%3AaOQAAOSwBLlVUEMB&LH_PrefLoc=1&_nkw=5mm+led+bezel&_sacat=0&_from=R40&rt=nc&LH_TitleDesc=0%7C0
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You're right. Wow, that's crazy cheap. Thanks.
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Quick question about laying out the board:
Moving from right to left, V1, V2, V3, V4:
Would it make most sense to layout the board with components for:
V1A: Preamp
V1B: Second gain stage
V2: Reverb driver
V3A: Reverb recovery
V3B: Tremolo
V4: final gain/phase inverter
What do you think? I'm going to dry run through it without soldering, see how things flow.
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This is how the Princeton Reverb is laid out. I would do it like this also.
My reasoning... Reverb recovery stage is dealing with a very small signal, probably the most sensitive stage in that amp. Keep the wiring tight and feed it directly into the final gain stage in the same bottle. Tremolo is a BIGGG signal. So is the PI. Put them in the same bottle.
V1A: Preamp
V1B: Second gain stage
V2: Reverb driver
V3A: Reverb recovery
V3B: Final gain
V4A: Tremolo
V4B: PI
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Thanks Sluckey. That makes a lot of sense. Really looking forward to checking out the Ampeg tremolo.
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... one thing I hope can be pulled off (but I don't know) is to find a single triode tremolo circuit that will work well in a cathode bias amp. Does one exist?...
The Fender Pawn Shop Excelsior has cathode-biased 6V6s and a singe-triode driven LFO that is buffered with an IRF820 MOSFET source follower
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I was looking that excelsior schematic the other day, can someone enlighten me how that circuit works, please.
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enlighten me how that circuit works
:dontknow:
the same as most amps, stick a small signal in the left side, it comes out the right side bigger and modulated at the trem freq, IF trem is turned on. I believe the PI is a cathodyne type. the trem modulates the post PI signals. It's self biased. guessing audio power ~~~~12-14w?
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Thanks for the other suggestions for the half tube. I think I'm going to go with the Ampeg tremolo. It looks very cool.
Anyway this is a question for Sluckey and anyone else who has an opinion. I started laying out the amp today using Sluckey's suggested layout:
V1A: Preamp
V1B: First gain stage
V2: Reverb driver
V3A: Reverb return
V3B: Second gain stage
V4A:Tremolo
V4B: PI
Here's the schematic:
https://imgur.com/a/xxOLgF2
I've gotten as far as the first two tubes. What do you all think of the routing/lead dress? Layout follows schematic except not shown is a mid pot at rear of the chassis, inserted between the tone/volume control and the input grid of the second gain stage; and a voltage divider not shown on the cathode of the reverb driver. Swapped it out from the original 1M resistor in that spot to reduce the reverb, which was pretty overpowering with the 1M.
Anyway, any suggestions or comments? Thanks!
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nice start!
might wanna update the schematic for the next poor service guy looking for a 1meg R :laugh:
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Ha! Yeah I think I will. Just haven't made changes on the fly.
See anything in there layout-wise that's going to cause issues w/noise or other problems?
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I've seen rats nest wiring quiet as a church mouse, perfect wiring sqealin like a pig. take some of discarded coating from stripped wire and use it on your longer leg parts, justin case ya lean in to much
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Well I finished wiring the amp and fired it up this morning after going over the schematic a few times. All looked good, found no errors but I'm getting silence. Am going to have to go over it until I find the wiring error.
All voltages are in the ballpark At 110V B+ at 367, plates at 360, 330 ohm cathode resistor reading 20 watts. So the tubes are dissipating about 85 percent or so. Grids at 320.
Power up and down the rails is good and is reaching the plates. All plates, cathodes getting more or less correct voltage.
The tremolo is oscillating correctly as the LED is flashing and the rate changes with adjustment of the speed pot and the fast/slow switch.
Thought there might be an error with the circuit so I grounded it out at the footswitch jack. No dice. Same with reverb; grounded it out and no signal as a result either.
Sluckey, one question: The "X" point coming off the wiper of the tremolo intensity pot should connect to the junction of the 220K resistors in the phase inverter, correct? I removed the ground for those two resistors and connected them to the wiper.
I wish I could do more with the amp at the moment but I'm headed out of town for the rest of the weekend. I took tons of photos and will go over them to look for obvious stuff. Haven't seen anything that looks wrong yet, no points grounded that shouldn't be (checked with multimeter) ... But someting is up! I'll get to the bottom of it.
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Do the pop test. (See Page 52 of Ch4 in Jack Darr book)
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Thanks for that link, looks like a great resource!
Before leaving I checked the all along the signal path with multimeter and also got Ac/DC voltages where I should be finding them. Probing produced no sound out of the speaker in any stage, and no trace of any sound with volume maxed. Did a bunch of probing for bad solder joints, all are tight. Checked output transformer, secondary and primary windings and they are fine. All tubes checked with a TV-10D/U before putting them in the amp and all are good. Eliminated speaker and speaker cable as well.
Am I correct in connecting the tremolo intensity pot wiper the junction of the 220ks feeding the output tubes?
Thanks!
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Am I correct in connecting the tremolo intensity pot wiper the junction of the 220ks feeding the output tubes?
yes
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> no trace of any sound
Put your ear IN the speaker. _NO_ sound? Then the speaker is not connected or heavily shorted. (Or bad speaker....)
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Just got the problem fixed: A solder joint that looked good but was open on the final gain stage. So, back in business and I'm getting volume, reverb and tremolo. Sweet!
One question: The tremolo appears to be overdriving, or having some other issue. From about 1 to 5 on the volume dial, it works great (and sounds fantastic). When I turn volumei up past that, I lose volume completely and hear only the thumping sound of the tremolo. Any suggestions?
Thanks!
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CORRECTION:
It's not a tremolo issue. Grounded out tremolo and the issue w/volume cutting out about halfway up the dail persists, along with a regular thump (even with tremolo grounded). Is this some sort of motorboating issue?
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Here's a more or less updated schematic. The only things missing from it are a mid control at rear, pre-phase inverter master volume and a three-way impedance selector. Thanks for any insight on this motorboating issue!
Ted
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> motorboating issue!
You have _four_ stages (lots of gain!) on _one_ B+ filter node.
Conservative design says no more than two stages on the same B+ filter. (This can be bent, but not in ignorance.)
Add a filter stage as shown.
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Thanks PRR. The suggestion is a great one. I did add another node and broke up the gain stage supplies. It helped to a degree but still got motorboating. I ended up dropping voltage to V1 from the supply down to about 115v. Low plate voltage, I know, but it's the highest I could get without motorboating, after subbing in lots of different resistors to test. At this point, with 115V on that first tube, the amp is responding well and smoothly. I thought it would be dirtier at such low voltage but it seems to be relatively clean ... in the neighborhood of what I was hoping for.
Specs on V1A and B:
Plates: 115vdc
Shared cathode resistor: 410 ohms
Cathode voltage: .5vdc
Thank you for your suggestion!
One issue I have to sort out is with the tremolo, which seems to be very powerful in that when I dime the intensity pot, I get a click with each pulse of the tremolo, when I apply signal from the guitar. I am going to experiment with some resistors, per a suggestion from Sluckey, to get that click to disappear since I am using the LED (very cool feature by the way)!
Thanks!
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> highest I could get without motorboating
Bigger caps are as effective as bigger resistors, but no added voltage drop. What uFd are you using?
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Thanks for responding!
Filter caps are 40/20/20/20
Cathode bypass caps in preamp and first stage, 22uf.
coupling caps throughout .022.
Just got some voltages put together. Biased right around 98 percent. Here's the full numbers:
B+ 360
B+1 296
B+2 221
B+3 204
Output plates 353/354
Output K (250 ohms) 18
Phase inverter Plate 164, K 42
Last gain Plate 138, K 1.2
Tremolo Plate 188, K 1.8
Reverb return: Plate 134, K1.2
Reverb driver: Plate 297, K6
Gain 2: Plate 78, K1.1 (12AY7). With 12AX7 around 115.
Pre: Plate 74, K1.1 (12AY7). With 12AX7 around 112
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Quick follow-up question to try to keep this going. At this point the main thing I'm wondering about is the effect of those V1 voltages on the 12AY7 tube. Does running the amp at these numbers risk harming the tube or causing reduced life and/or other issues?
These voltages have eliminated the motorboating but I don't know what effect they will have long term.
Thanks for your insight!
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I ended up dropping voltage to V1 from the supply down to about 115v.
How did you do this?
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I soldered a 15k resistor to the junction of the two plate load resistors at V1 (82K and 100K). Then I attached the power supply to the other side of that resistor. That brought the voltage down to where it is. Any lower value on that resistor and I got motorboating, so that was the minimum number that seemed to do the trick.
That 115 is the plate voltage with a 12AX7. With an AY it's about 80 or so.
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I soldered a 15k resistor to the junction of the two plate load resistors at V1 (82K and 100K).
You also need a 22µF filter cap at the junction of those three resistors.
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Thanks Sluckey. Will that last cap serve to increase voltage to V1?
Coincidentally this is the fourth of this circuit that I have built but the first to have this motorboating issue, and the only difference between it and the others is the insertion of the Ampeg tremolo circuit and the removal of one channel to free up a triode for the tremolo. A few people had suggested to me that the tremolo was the root cause but it isn't; to make sure I completely removed the tremolo from the circuit by removing the jumper from the intensity pot wiper to the junction of the two 220K resistors feeding the power tubes, and running those straight to ground as in the original, and also removed the power feed to the tremolo triode's plate. Even with those two tie-in points to the circuit removed, I still got the motorboating issue so it isn't that.
Some had suggested that I had too much overall gain in the amp and that was causing the motorboating, but I wonder about that as previous versions of this amp have had no motorboating issues, with the same gain stage layout.
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most of my builds use a AU7 in V1 running between 70 - 90vdc at plate, NO problem.
you're starting with a mV signal, swinging it up to a volt or 2, there's no reason you need 200+vdc to swing such a small signal. You DO want the B+ stable, small AC on the DC will make a big headache
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Thanks shooter, that's good to know. So definitely then per Sluckey's instructions run a 22uf cap to ground from the three-resistor junction to keep the AC out, and I should be good to go on that triode? Thanks. How about the bias? Obviously V1 is cathode biased so its self-regulating (doesn't need to be fixed) but if I'm doing the math right it looks like I have super low dissipation on that tube. Is that an issue?
Not noted in schematic but I replaced the shared cathode resistor in V1 from the original 820 to 560.
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The cap puts that new B+ node at AC ground just like any other filter cap. Without the cap the signal at the plate of V1a and the signal at the plate of V1B are mixing together and will cause problems.
Not noted in schematic but I replaced the shared cathode resistor in V1 from the original 820 to 560.
:huh: WHOA!!! Are you saying that V1A and V1B are sharing a cathode resistor? That's a big no-no and is likely the cause of the motorboating. You cannot allow two consecutive gain stages share the same cathode resistor. Give each triode it's own 1.5K and 22µF and I bet the motorboating will go away.
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Yes, they do share the same bypass cap/resistor.
Previous version DID not as I had one channel voiced differently than the other. I figured I would save space on the board this time by doing a shared cathode.
I'm learning this stuff as I go and did not know that was a no-no in successive gain stages. That is very good to know!
Now to sneak away from work while nobody is looking and see what I can do. Thanks!
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Let us know how it turns out. I still recommend using the extra B+ node as you have it now, but add the filter cap. And you can probably reduce that 15K to 4.7K if you want a little more voltage.
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PRR made this post a few years ago...
https://el34world.com/Forum/index.php?topic=11053.msg101896#msg101896
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Success!
I split the cathodes on the preamp and first gian stage, and took out that last 15K resistor (that I only put in because it was the lowest value that would stop the motor boating). So with cathodes split and pre and first gain plates running directly to original power supply node, the motor boating is gone. With the 12AY7 I have about 105-110V on the two plates.
Thank you both very much for the suggestion, and for the link to the previous thread about this. I didn't know a feedback loop was the result of sharing those cathodes. It's just nothing I had ever come across before, and had no idea was an issue.
Getting back to the power rail: I am sure I need to adjust it as I have four plates on that last node. How would you suggest breaking it up? Moving one of them to the node that feeds solely the PI?
And/or do you still think I need a fifth electrolytic tied in at the last spot?
Many, many thanks for helping get to the bottom of this. Makes my day.
EDIT: Sorry, in photo forgot to addd the values of the resistor. From left to right they are 5K, 18K and 5K. I may play around with those once I get the node situation figured out, to get the voltages back to what specs say.
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How would you suggest breaking it up?
You already had it. But you NEED the additional filter cap. And reduce the size of the 15K to 4.7K.
DON'T USE THE PI STAGE B+ NODE FOR THOSE PREAMP OR REVERB RECOVERY STAGES.
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Just for a bit of info... The very popular and successful Fender AB763 amps supply SIX stages with the same B+ node! I always thought that was a bad idea and would never design anything like that. But it served Leo quite well.
My point… It's a better idea to split up the B+ nodes, but it sure ain't necessary. I don't mind spending the extra few dollars to have a 'better' B+ rail. Now, you decide what you want to do. :wink:
On another note... Have you reconnected the tremolo? Did you use the resistor suggestion? Thumping gone?
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Sounds good. I will add an additional 22uf at the junction of the plate load resistors feeding V1, and move the PI stage from node D to node C.
One thing I'm not following: You mentioned reducing the 15K resistor to 4.7K.
If you are referring to the one I previously added to the junction of the V1 plate resistors and from there running to the power supply, I already took it out when I split the cathodes. So, V1 plates now run directly to Point D on the drawing of my power supply.
As for the tremolo ticking yeah, it's pretty much gone. At very high volume and with the feedback switch set in Fender 5E3 mode (no feedback, 22uf cap in circuit) I still get very slight, slight ticking when I have all the dials pegged. You'd suggested a 470K resistor to the intensity wiper to start. I tried that and moved up until I hit 1M. With that one in, it's still very, very slightly there but only with volume dimed.
Thanks for the suggestions!
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I don't mind spending the extra few dollars to have a 'better' B+ rail. Now, you decide what you want to do. :wink:
I couldn't care less about what it costs. I want it done right, always! That's my only consideration. Thanks!
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fwiw;
when I tweak my B+ rail, I ballpark the voltages based on expected signal, so V1 doesn't need a lot of plate vdc, the PI does. I also like a tight NO sag rail, so my 1st filter cap(S) tend to be ~~100uF, I add to each node when I want to get ripple vac down even more. I build xSE so anything more than ~~2vac at the 1st node will probably cause some hum. My V1 node I like to try and get <10mVac idling.
On PP amps the PA gives you a natural cancellation of ripple because the tubes are outta phase
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Duncan's PSUD2 can be fun to mess around with: http://www.duncanamps.com/psud2/
Multiple stages of filtering isn't just for stage separation, it's the best way to reduce noise within your cap space/budget. Before using that software I didn't really understand what an RC filter does compared to connecting something straight after the rectifier... even if the rectifier is directly followed by huge cap you get an obnoxious "sawtooth" ripple which is smoothed to a sine wave with even one added resistor/cap. Yes, PP output stages can sometimes be used unfiltered.
fwiw;
when I tweak my B+ rail, I ballpark the voltages based on expected signal, so V1 doesn't need a lot of plate vdc, the PI does. I also like a tight NO sag rail, so my 1st filter cap(S) tend to be ~~100uF, I add to each node when I want to get ripple vac down even more. I build xSE so anything more than ~~2vac at the 1st node will probably cause some hum. My V1 node I like to try and get <10mVac idling.
On PP amps the PA gives you a natural cancellation of ripple because the tubes are outta phase
Yes, PP amps cancel ripple fed into the output stage but as you know, ripple in the preamp will be added to your guitar signal and you'll hear it no matter what output type is used! So you can get away with not filtering before the OPT center tap but doing some filtering there (even as low as 50ohms/100uf if you don't want to lose much voltage) is a good opportunity to begin to lower the ripple which needs to be low by the time it hits the preamp.
You don't need to go this big, but caps can be pretty affordable these days if you're willing to wirewrap/solder "snap in" PCB caps. Here's 470uf, 450v for under $7: https://www.mouser.com/ProductDetail/United-Chemi-Con/ESMQ451VSN471MR45S?qs=sGAEpiMZZMvwFf0viD3Y3Wd4FpOoUsJZFxfSsu%252bAVZQ%3d
So despite everything I said about designing your stages correctly, overkill is also an option :icon_biggrin:
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Please don't put a big ass cap on that 5Y3! You have sufficient filtering already for that amp. You only need to shuffle nodes a bit.
Your latest schematic only shows 3 nodes, but your hand drawn pic shows 4 nodes. I can see a JJ 40-20-20-20 can on the chassis and that's all you need. Just wire it like this attached schematic and fix your schematic or call this one the final.
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> The very popular and successful Fender AB763 amps supply SIX stages with the same B+ node! I always thought that was a bad idea and would never design anything like that. But it served Leo quite well.
me> Conservative design says no more than two stages on the same B+ filter. (This can be bent, but not in ignorance.)
We agree. Using "free" time to build ONE amplifier, simple rules like "2 stages per B+ node" keep us out of trouble. When you know you are going to sell tens of thousands, then you get a sharper pencil. Interstage losses "might" exceed forward gain and B+ node sneak-back, and allow you to save thousands of pennies.
BTW, I did know we had 1st and 2nd preamp AND reverb recovery on the new B+ node I proposed in reply #30. 3 stages. Bad? It's a tricky point, I think it is allowable, AND it won't motoboat until Reverb is turned-up so can be diagnosed with one finger.
I did NOT know two *sequential* stages had the SAME cathode network. That was NOT on the "more or less updated schematic". I hate getting lost in Brooklyn because I'm looking at a map of the Bronx.
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Please don't put a big ass cap on that 5Y3!
:BangHead:
fwiw, I never use tube rectifiers, well, once, the rest SS
:think1:
for those lurking n learnin, you have available here the top shelf overlords, I did well for 30yrs fixin very complex systems, my "ace in the hole" were guys like Sluckey, PRR, HBP, and more. they are the "notch above" we aim for
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I did NOT know two *sequential* stages had the SAME cathode network. That was NOT on the "more or less updated schematic". I hate getting lost in Brooklyn because I'm looking at a map of the Bronx.
:icon_biggrin: Took a long time for that little nugget to surface! Coulda saved a bunch of time and head scratching if that had been on the schematic. Coulda saved a bunch more if there had been 4 power supply nodes on the schematic too.
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Please don't put a big ass cap on that 5Y3! You have sufficient filtering already for that amp. You only need to shuffle nodes a bit.
Your latest schematic only shows 3 nodes, but your hand drawn pic shows 4 nodes. I can see a JJ 40-20-20-20 can on the chassis and that's all you need. Just wire it like this attached schematic and fix your schematic or call this one the final.
Thanks. Yes at this point it's changed a ton. I'm trying to update it and will post it when I do. Thanks!
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Thanks for the suggestions ... I've seen and built a bunch of simple amps using 40uf with a 5Y3 but I am no expert. Though I haven't killed any 5Y3s yet, I don't want it to happen ever and am always willing to take advice. I've got a bunch of 33uf/500v Nichicon caps and there's plenty of room to put one of those in the first stage, and take that 40 out of there. Thanks for the guidance.
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I don't consider a 40µF to be a big ass cap for a 5Y3. Some others were talking about 100µF and even 470µF. Now those are big ass caps for a 5Y3!
If you are concerned about the 40µF being connected to the 5Y3, then just put one of the 20s on the 5Y3 and move the 40 to node D. No need for additional caps.
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Thanks Sluckey. Since reading the post this morning I have obsessively read a million varying opinions on how much is too much in the first filter. I'll take your word for it.
One thing I may do anyway is just use a 5U4 in place of the 5Y3. The overall B+ voltage in the amp is a bit lower than in the past few versions of this build (different power transformer in this one than in the others) so I think there's room for more voltage as long as I adjust the bias. The 5V winding is rated for 3A so it shouldn't be a problem and unless I'm wrong the 5U4 can handle a larger first filter.
Again, thanks for your expertise. I am learning a ton on this build.
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GZ34 would be my choice of rectifiers. They have an indirectly heated cathode. That means it takes about 20 seconds for the B+ to come up after you turn on the power. 5U4 comes up in about 5 seconds. I like the slow warm-up.
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Thanks, I actually have some nice NOS old Mullards. I tried one in the circuit but it just pushed the B+ too high (around 445 B+ or so). With the 5U4 I'm getting around 405-410 or so. I would probably use one but I don't have any zener diodes to bring the B+ down (plus I heard they're noisy. Any opinions on that actually?)
What I do have are some CL-80 inrush limiters. How about putting one or two of those on the AC line in conjunction with the 5U4?
Thanks!
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I don't consider a 40µF to be a big ass cap for a 5Y3.
Same.
Leo ran '32uF' in the stock 5F2A.
I used 40uF with an old 5Y3G in my first 5F2A - thing was still going strong years later last I heard.
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I have a nice stash of old stock RCA 5Y3GTs. No modern stuff, same goes for power and noval tubes.
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The published ratings on tube datasheets are typically conservative IMHO, because the manufacturers wanted to ensure that people would operate tubes within parameters that would help ensure that manufacturers weren't inundated with warranty obligations.
In reality, tubes are pretty tough and can take a fair bit of 'abuse'. I say this with the caveat that (of course), the harder you run a tube, the shorter it will last, generally speaking. But running it a 2 x the manufacturers recommended reservoir capacitance rating shouldn't do too much harm to the rectifier tube. Of course, you can enhance the life of the tube by making sure the envelope is well-ventilated and not within too-confined a space. The 'work' done by the rectifier tube's plates - to pull electrons out of the reservoir cap (and other filter caps') positive poles - increases as the capacitance is increased, because your tube is having to 'evacuate' more electrons (per-unit-of-time) from the positive poles of the filter caps. So all-in-all it will run hotter, and therefore the more thought you put into planning to allow for physical space around the envelope to help with the heat dissipation, the better.
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Thanks for the food for thought.
I just finished building the cabinet yesterday (head only) and tried to cut the back panel as open as possible to keep the air flowing. Don't think I can get more open space in there without making it too flimsy. We'll see!
One other question:
I have since adjusted the power supply caps so the first cap's 20, then 20, 20 and finally the 40 which now feeds the first two gains and reverb recovery.
I previously had had some issues with a slight ticking of the tremolo that occurred when I dimed the volume and put the negative feedback switch into the stock Fender 5E3 position (with 22uf bypass cap in circuit). The tick would occur with the rate of the tremolo, and not constantly; only when a note was hit. Before fixing the shared cathode issue on the first gain stages and shuffling the filter caps I was able to get rid of the ticking for the most part by running a 1m resistor into the wiper of the intensity pot.
Now with the new filtering lineup, the ticking is back, but a bit stronger. I don't want to increase the 1m resistor any more because it attenuates the tremolo effect. Any other suggestions for getting rid of the tick?
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I was able to get rid of the ticking for the most part by running a 1m resistor into the wiper of the intensity pot.
That resistor is supposed to be on the top of the pot, not the wiper. Wire it like the attached. Experiment with the cap across the pot.
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Ahha!
By the way, the tremolo is amazing! It's got more of a pronounced 'thump' than the Fender tremolos I've heard. And so full and thick. Really, really nice. And from half a tube!! Amazing, great stuff. Thanks Sluckey.
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the tremolo is amazing!
I love stories with happy endings! :thumbsup: :thumbsup: :thumbsup:
Sluckey, I continue to have a great deal of admiration and gratitude for your remarkably generous help to this forum and it's members. You're a stellar guy! THANK you so much for your contribution and helping those of us (myself in particular) who are lost in the fog in figuring this stuff out.
It's been fun watching this thread evolve into another Sluckey assisted success story! Bravo!
With respect, Tubenit
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I continue to have a great deal of admiration and gratitude for your remarkably generous help to this forum and it's members. You're a stellar guy!
I second that!
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It's been fun watching this thread evolve
The student is a WILLING participant, that always helps :laugh:
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OK, so I have one more final, unrelated question (for now lol).
So, the output transformer has 4, 8 and 16 ohm taps. I have switchable negative feedback that runs into the speaker jack. It's 47K which is recommended by Rob Robinette for an 8 ohm load; should I just leave it wired to the jack and make no allowances for when switching to a 4 or 8 ohm load?
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Hardwire the NFB to the 8Ω tap.
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Thanks Sluckey.
Thought I'd post a few photos of the amp which is more or less done. It's for a giant Big Lebowski fan (thus the "Achiever" and "Abide" on the guitar jack). Evil Twin (I'm a twin) is my hobby name/Facebook handle.
I moved the LED off the front panel and put it at the rear (not shown yet in pic) because it was just too bright and distracting up front. The blue glow emanating from the back is cool though. I put the fast/slow switch up front where I'd originally put the LED.
Had some friends over and we played the amp for about four hours last night with no issues. Very quiet, sounds excellent. Originally the tone was not too bright but I added a 500pf cap across the 680K resistor running out of the preamp. That gave a much better range of high end on the tone control. Amp sounds killer with really nice bass response. Great reverb and the tremolo is trimmed up well now. No more ticks.
One question: After a few hours the power transformer was HOT. I mean, hotter than any amp I've played/built etc. I know it was on for many hours on a muggy, warm night (we were outside) but man. Worried me a little bit. I ended up putting a small house fan behind the amp to keep some air moving over it late in the night. It's got me wondering about whether I should add a small AC or DC-powered cooling fan. What do you all think about putting in a small fan? A full mains AC-powered fan seems like it would be easier to deal with, wouldn't have to rectify AC to make it work ... I do have an unused 50V bias winding on the power transformer that I could use if I could figure out how to rectify the power to the correct DC.
THoughts?
Again, thanks everyone for the help!
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Originally the tone was not too bright but I added a 500pf cap across the 680K resistor running out of the preamp.
First time noticing that 680K resistor. Where did you get that idea? Why? You won't find a resistor there on the original 5E3.
After a few hours the power transformer was HOT. I mean, hotter than any amp I've played/built etc.
Cathode biased amps are supposed to run hot. They are biased to run full throttle even when idling. You can cool it down some by using a 500Ω/10W cathode resistor on the output tubes. I've never felt the need for a fan on a 15 watt amp.
If you must use a fan then 120VAC is the easier circuit, but they may be too noisy for you. 12VDC computer fans are much quieter and cheap. Get one of the Geek Squad guys let you go dumpster diving behind Best Buy. A small bridge rectifier and 470µF cap connected to the 6.3VAC filament string will run the 12VDC fan at reduced voltage. This will be really quiet and push enough air.
I do have an unused 50V bias winding on the power transformer
Tell me everything you know about this winding. I suspect that rather than a real separate winding you simply have a tap on the HT winding that's not rated with enough current capacity to run a fan. If so, don't mess with it.
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Yes, sorry, it’s just a tap.
Cathode resistor is currently 415 ohms/10w and dissipation is around 90 percent. Thanks!
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we were outside
how long was the extension cord
I've found some cheap 100'rs heat up my SE amp(40W) but the tranny has 100mA of "unused space" so she just runs warmer :icon_biggrin:
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Not too long, maybe 15 feet. We were out on the back deck👍
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About the 680k...
When I built the first version of this a few months ago I didn’t account for the gain drop from sticking the reverb circuit in. So another gain stage wdnt in (the one I infamously shared cathode resistor with first stage) but that made it too grainy... amp was breaking up around 2 on the dial. Putting the 680k resistor in after the coupling cap brought it down just about right.
About the heat:
Most of the stuff I’ve done is cathode biased but it just seems like this one runs hotter.. I usually use ClasscTone 40118016 (330-0-330@120ma) for this amp but the transformer in this amp was given to me so I put it in. Don’t know much about it but the hv secondary has identical ratings to the CT.
https://www.amplifiedparts.com/products/transformer-fender-replacement-power-330-0-330-v-120-ma
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Hi all,
Just two quick follow-ups on the tremolo circuit Sluckey suggested and that I ended up putting in the amp. Sounds amazing!
1. The foot switch works as intended and kills the tremolo. The only minor issue I'm having is that this does not darken the LED. It stays on even when switch turned off.
So would there be any issue with killing it a different way, using a DPDT switch? One half would kill the voltage to the plate; the other would switch the junction of the two grid leak resistors between ground and the wiper of the intensity pot. Seems like this would completely remove the tremolo from the circuit. Any thoughts?
2. Second, minor question: The intensity pot doesn't really do anything until I hit about six on the dial. Below that, there is no pulsing but above it, the trem effect kicks in. Maybe run a 1m across the pot to turn it into a 500K as opposed to the 1m I'm using?
Thanks!
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1. The foot switch works as intended and kills the tremolo. The only minor issue I'm having is that this does not darken the LED. It stays on even when switch turned off.
Does that really bother you? Put the foot switch between the LED cathode and ground. The LED will be dark when the switch is open and it will pulse when the switch is closed. This also means the footswitch must be attached for the trem to work. Unless you use a switched jack to provide the ground to enable the LED if you unplug the footswitch.
2. Second, minor question: The intensity pot doesn't really do anything until I hit about six on the dial. Below that, there is no pulsing but above it, the trem effect kicks in. Maybe run a 1m across the pot to turn it into a 500K as opposed to the 1m I'm using?
Sounds like you may be using a pot with a log taper? If so, change to a liner pot. Any better?
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D'oh, yes I'm using an audio taper pot; don't have a linear pot at 1M. I'll live with it for now :)
The LED is awesome and very cool, but it does get a bit bright so I'd like to have it off when not being used. No big deal either way, I can live with it.
Thanks!
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Just move the footswitch as I suggested and the LED will be off when the trem is off. Can't say for sure but the trem may be slow to start up. Easy to do and undo if you don't like it.
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Thanks Sluckey. Such a great tremolo, I appreciate the help.