Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dwinstonwood on August 01, 2020, 10:18:17 pm
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Hey everyone, while I'm waiting on my 6G3 cabinet, I can't seem to stay away from imagining another project. I figured it was maybe time to think about 6L6's.
I've had an interest in the Blonde and BF Bassmans for a long time. I'm sure this has been done many times, and much better, but I got the idea to draw up an AA864 with one channel. It would use the three gain triodes of the Bass channel, but use the components/values of the Normal channel for guitar.
A major issue for me is the availability of a pre-punched chassis. I have a drill press and step bits, but making a custom chassis is a big task that I don't enjoy.
It turns out that the Mojotone 6G2 Princeton chassis is just right. I would only need to install an octal to 9-pin adapter plate for the PI tube. I would use the other 6V6 hole and the 5Y3 hole for the 6L6's, since the AA864 has SS rectification. The extra pot on the front could be used for a Presence control, etc. The ground switch on the back would become a standby switch.
Here's a layout I've been working on, even though I don't need another amp.
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even though I don't need another amp.
:laugh:
I'm not a guitar player, but I always seem to have at least 2 amps ready, just in case :icon_biggrin:
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A 6L6 Express then :icon_biggrin:
From a technical perspective, standby and ground are equally redundant.
Whereas a switch at that end of the amp could be used to provide an alternative response characteristic, eg HT sag resistor, or, if you can source a PT that provides a proper HT that’s closer to 400 than 450Vdc, cathode bias.
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I'm not a guitar player, but I always seem to have at least 2 amps ready, just in case :icon_biggrin:
Who knows, you could wake up one morning with the ability to instantly visualize every possible chord inversion on the entire neck.
A 6L6 Express then :icon_biggrin:
From a technical perspective, standby and ground are equally redundant.
Whereas a switch at that end of the amp could be used to provide an alternative response characteristic, eg HT sag resistor, or, if you can source a PT that provides a proper HT that’s closer to 400 than 450Vdc, cathode bias.
:icon_biggrin: I like 6L6 Express. I was thinking of a play on Bassman - maybe, "Guitarist." :laugh: Pretty bad, I know.
That's good to know about standby switches. In my very limited understanding I assumed that with SS rectification standby was important to protect the tubes. :dontknow:
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Who knows
I use the "dave" method;
yup, if I put my fingers here, sound pretty good - memorize
strum here with fingers there, yup, sounds good, memorize
as a climber you practice so much that muscle memory gets programmed
as a guitar player, seems to work about the same :icon_biggrin:
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So, I'm drawing up the schematic for my single channel AA864, and the bass channel - which I'm using with normal channel components - has this circuit (see image) right before the last triode before the PI.
I'm guessing that this is a Band Pass Filter, but I really don't know. :dontknow:
If it is, the HPF has a cutoff of 7.23Hz and the LPF has a cutoff of 723.43Hz. If I'm seeing this correctly, it seems like a narrow band, which I guess makes sense for a bass.
OTOH, if I've got it all wrong, what is this circuit doing? Should I just get rid of the second/lower 220K resistor and the .001uF capacitor?
Thanks, as always!
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> 723.43Hz. If I'm seeing this correctly, it seems like a narrow band
Does it sound dull?
The high-cut may be closer to 1400Hz.
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... draw up an AA864 with one channel. ...
It turns out that the Mojotone 6G2 Princeton chassis is just right. ...
Here's a layout I've been working on, even though I don't need another amp.
Does your layout draw each part to-scale? If yes, what is the overall length of your board? Does it fit in the space afforded by the 6G2 chassis? While still leaving room all around the circuit board (to avoid being too cramped)?
I've found it's very easy to breeze past the basic requirements of "Will it fit?" when drawing up a new idea.
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Thanks HotBluePlates,
Yes, I've been laying this thing out to fit in a 17" chassis. The 6G2 is 17 1/4" long x 7 18" wide. My board is 11 1/2" long. But, there are other options, too, like some of the Marshall chassis. With those I can use multicaps for filtering instead of a Fender doghouse setup. I thought about the Hoffman Stout chassis, but I'd need to redesign my board; it's only 12" wide.
I'll post a revised layout and a schematic tonight.
Pdf64 suggested "6L6 Express" as a name, and looking at the Trainwreck Express schematics there are similarities. It might be interesting to try some of those component values instead of the BPF I highlighted above. For example, the Trainwreck schematics suggest values like .001uF and 68K before the third triode. Or, .002uF and 150K, etc.
Then, again, I could just try building an Express!
https://el34world.com/charts/Schematics/files/Train_wreck/Trainwreck_express.pdf
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Here's the schematic and an updated layout. I might still change some things before ordering parts.
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Hi, I'm still mulling over this project (and raising the funds), and I have a question, or two.
In the red box in the attached image is what I think is a voltage divider that results in 50% attenuation. Could those two 220K resistors be replaced with a 500K linear pot to act as a gain control?
Also, - and this is where serious confusion sets in - I'm under the impression that at least three things are going on here:
1.) a voltage diver
2.) a high pass filter (with maybe some other frequency filtering, too)
3.) and, the setting of Zout and Zin between the two stages.
Since all of those things seem, as I understand it, to be interconnected and interdependent would using a pot have an adverse effect? This is an area of amp circuits that I'd like to get a handle on, and understand better.
Thanks!
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In the red box in the attached image is what I think is a voltage divider that results in 50% attenuation. Could those two 220K resistors be replaced with a 500K linear pot to act as a gain control?
It is a voltage divider and you can use a pot to dial in some more gain.
Also, - and this is where serious confusion sets in - I'm under the impression that at least three things are going on here:
1.) a voltage diver
2.) a high pass filter (with maybe some other frequency filtering, too)
3.) and, the setting of Zout and Zin between the two stages.
1. Yes, a voltage divider.
2. No, it's a low pass filter. Remember, it's a bass amp. I would remove that cap if this amp will be used for guitar.
3. Impedance is not really an issue here.
Since all of those things seem, as I understand it, to be interconnected and interdependent would using a pot have an adverse effect? This is an area of amp circuits that I'd like to get a handle on, and understand better.
No problem. I suspect the voltage divider was all about balancing/matching the bass channel output with the Normal channel two stage output.
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Thank you sluckey! I'll add the pot into my schematic and ditch the cap (I assume you're referring to the second .001 cap). :thumbsup:
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> a high pass filter
The 0.1uFd and the 440k of resistance is a 4Hz high-pass (low-cut) filter, yes. 4Hz is "nothing"; it's really just blocking the DC.
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So, I'm drawing up the schematic for my single channel AA864, and the bass channel - which I'm using with normal channel components - has this circuit (see image) right before the last triode before the PI.
I'm guessing that this is a Band Pass Filter, but I really don't know. :dontknow:
If it is, the HPF has a cutoff of 7.23Hz and the LPF has a cutoff of 723.43Hz. If I'm seeing this correctly, it seems like a narrow band, which I guess makes sense for a bass.
Yep - its a band pass filter
1) Bass rolloff = 1/[2Pi.100nF.440k] = 3.6Hz @ 20dB/decade (which is nuthin' much)
2) Overall attenuation from the voltage divider = 220k/440k = 0.5 (-6dB)
3) Treble rolloff = 1/[2Pi.1nF.(220k||220k)] = 1,446,281Hz* @ 20dB/decade (1.4MHz* Oops - not 1.4Khz - didn't put enough zeros in my calculator :l2: )
It interacts with the (~40K) output impedance of the previous stage and the (~100pF) input capacitance of the following stage (so gain and treble rolloff point will shift slightly lower)
or in 'geetar amp speak' its a hard-wired treble control with the treble knob permanently turned down a bit with a 1nF to ground to slug resistor noise from the 200k 'grid stopper' - and that's my story and I'm stickin' to it
:m8
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> a high pass filter
The 0.1uFd and the 440k of resistance is a 4Hz high-pass (low-cut) filter, yes. 4Hz is "nothing"; it's really just blocking the DC.
Thanks PRR. Yep, that's what I was looking at when I posted "high pass filter." I forgot about the second 220K resistor and the .001 as a second filter, i.e., a low pass, as sluckey pointed out. Although, oddly enough, in Post #5 (two months ago) I did comment on it! :w2: Not surprising.
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Thanks tubeswell! I knew there was more going on there than I could see. :icon_biggrin:
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It interacts with the (~40K) output impedance of the previous stage and the (~100pF) input capacitance of the following stage (so gain and treble rolloff point will shift slightly lower)
Now I remember an Uncle Doug video where he said you can think of a 12AX7 as a +/- 151pF capacitor. So, I can see how the 220K grid stopper and the tube would together act as a low-pass filter, in that respect.
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I think in more simplistic terms. That .001µF cap connected directly to the grid, is gonna dump a shitload of highs directly to ground. That's why I said its a low pass filter. I could have said it's a treble bleed cap. Same thing in that circuit. IMO no need to break out a calculator for that.
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It interacts with the (~40K) output impedance of the previous stage and the (~100pF) input capacitance of the following stage (so gain and treble rolloff point will shift slightly lower)
Now I remember an Uncle Doug video where he said you can think of a 12AX7 as a +/- 151pF capacitor. So, I can see how the 220K grid stopper and the tube would together act as a low-pass filter, in that respect.
Cin = C (grid to cathode) + C (plate to grid) x A (where A = voltage gain of the stage. For a 12AX7 with a 100k plate resistor, and a fully bypassed cathode resistor Voltage gain is about 60)
So 1.6pF + (1.6pF x 60) = 97.6pF
Uncle Doug would've assumed a voltage gain that was somewhat higher - goodness knows what he was thinking
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............. = 97.6pF...
I pencil it as 100p.
160pFd sounds wrong.
"0.001uFd" is 1,000pFd. So another 100pFd on top is a small difference. Without the 0.001uFd it might be flat to 14kHz, flat for any guitar purpose. (Too flat, because the very high-order harmonics of a string are nasty, even by rock standards.)
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I think in more simplistic terms.
I honestly try to! :icon_biggrin: The clearer this stuff becomes, the easier that is.
In that respect, I'll use just two components: a .1uF blocking/coupling cap connected from the preceding plate to one side of a 500K linear pot; the other side of the pot connected to ground; and, the wiper connected to the next grid.
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Here's the final version. I'm sure there are mistakes.
It will still fit in a 6G2 Princeton chassis. Same number of knobs - volume, treble, bass, gain instead of volume, tone, speed, intensity. I'll change one of the octal holes to 9-pin with an adapter plate. Might need to enlarge the PT opening. I'll maybe make this one as a head.
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Will make a fine raucously loud geetar amp
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are the power transformer and output transformers suitable for a 50W amp going to fit that chassis without opening up / re-drill mounting holes? or did miss a mention about something other than 6L6 for the output stage?
6G2 PT and OT are much smaller than AA864 parts.
6G2 PT dimensions - http://www.classictone.net/OOA-85282_Chassis_Cutout_C-100.pdf (http://www.classictone.net/OOA-85282_Chassis_Cutout_C-100.pdf)
AA864 PT dimensions - http://www.classictone.net/OOA-85285_Chassis_Cutout_C-400.pdf (http://www.classictone.net/OOA-85285_Chassis_Cutout_C-400.pdf)
6G2 OT dimensions - http://www.classictone.net/40-18045.pdf (http://www.classictone.net/40-18045.pdf)
AA864 OT dimensions - http://www.classictone.net/40-18008.pdf (http://www.classictone.net/40-18008.pdf)
--pete
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Will make a fine raucously loud geetar amp
:icon_biggrin: That's pretty much what I'm thinking. Thanks tubeswell!
are the power transformer and output transformers suitable for a 50W amp going to fit that chassis without opening up / re-drill mounting holes? or did miss a mention about something other than 6L6 for the output stage?
--pete
Probably not! It just seems to have the right number of holes. :icon_biggrin:
But, you are right. It should have a larger chassis that will not only handle the bigger tx's, but allow for the extra filter caps, too (like maybe room for a doghouse).
This definitely requires more serious planning before I buy anything. Thanks pete!
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Consider having a custom chassis made.. https://seasidechassisdesign.com/
and I had one made here https://www.facebook.com/synapticamps/?ref=page_internal http://www.synapticamps.com/ (https://l.facebook.com/l.php?u=http%3A%2F%2Fwww.synapticamps.com%2F%3Ffbclid%3DIwAR1-AhTpzkvyBYhnrZONZhepKGz6vJ-oMIXLr1XrNT0h5a7vPilJss2f6MY&h=AT2Bjp34dXZ16Yas9oWzxU8iBPrdR6Lm6RZHvEA74D2iKwfjDB5xcsKqozDh2xSXXG0GoaKxDR1we_aA86fWFAkP5Y_N-cPMksqnmf7P5qWBM_Lov960An3Unv86W418A8IxNA)
and both do excellent work.
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Thanks mresistor.
An AB763 chassis is closest to the original AA864 size, although the chassis I just used - the 6G3 - would also work and is a bit shorted.
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https://www.ebay.com/itm/Modified-JTM45-16Ga-Gal-steel-20X8X2-1-2-inches/223193599066?hash=item33f7600c5a:g:HasAAOSwVW9bx4nT
that linked above would work - build a marshall style combo, without the typical marshall circuitry. :icon_biggrin:
--pete
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Thanks DummyLoad. I like the idea of an amp with the tubes right-side-up.
On a different note, I found this online PT calculator. I'm sure it's not exact:
https://thesubjectmatter.com/calcptcurrent.html
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You could put that amp in Hoffman's Stout chassis.
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You could put that amp in Hoffman's Stout chassis.
And, the price would be hard to beat! Thanks.
One question: will the multi-cap hole on the end accept an 8-pin socket (in case I wanted to go GZ34)?
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yes
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Your amp might look like this if it were built in Hoffman's Stout chassis...
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Wow, I feel honored that you put that together sluckey.
I'm studying it now. Your organization and grouping of components is super-logical and compact. I would not have been able to come up with anything near that sensible. Thank you!
Well, I no longer have an excuse for not starting this project. :icon_biggrin: So, it looks like I'm going to have another guitar amp. And, they keep getting louder! AC4 > 6G3 > AA864.
Much appreciated!
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I put this together just as a mind exercise. You may not like having that much amp in a tiny box. I kinda like it though. I've built several amps in that size chassis.
If you want to pursue this I'll put together a document file just like all my other documents. It will include your schematic, this layout, and a full size drill template to make drilling and staking the board easy. LMK.
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That's very generous sluckey! I think that sounds like a good idea.
I guess the big question is, do I get a link on this page? :icon_biggrin:
https://el34world.com/charts/TubeAmpSchematics.htm
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Sure... as soon as you build it. :wink:
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Here's the document file. I've compared the layout and schematic several times. I don't see any errors. Doesn't mean there aren't any. I'm confident enough with the layout to build this if I wanted one. I do suggest you compare the schematic and layout looking for errors but mainly to get familiar with the layout. If you spot any errors or just have questions, speak out. Revisions are easy.
EDIT... removed attachment
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Many thanks sluckey! Your layout takes care of the of the hardest part (for me) of the design work.
Also, thanks for improving the rectification and B+ filtering on the schematic.
I like the push/pull volume pot for a bright switch. The only thing I can think of at the moment is to buy an OPT with multiple speaker ohm leads, and add a switch, or extra jacks (not sure which is better). My plan is to build it as a head, so that would be a good feature to have.
I'll spend some time tonight going over things and write up a BoM!
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Take a look at this project...
http://sluckeyamps.com/phoenix/phoenix.htm
It's the same power amp you have. The Hammond 1760J is perfect for your project too.
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Very cool. Your writeup and photos are worth a great deal.
The Hammond 1760J it is, then.
Hammond lists the 290EX as their replacement for the 125P7D:
https://www.hammfg.com/files/parts/pdf/290EX.pdf
Or, the slightly more expensive Classic Tone 40-18005:
http://www.classictone.net/40-18005.pdf
But, I've had good luck with Hammond, so far, I'll stick with theirs.
Hoffman sells this 6A/125V rotary:
https://el34world.com/charts/images/rotarydiagram2.jpg
Maybe, similar, or the same, that you used?
And, where did you get those cool, black tube shields? :icon_biggrin:
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Very cool. Your writeup and photos are worth a great deal.
The Hammond 1760J it is, then.
Hammond lists the 290EX as their replacement for the 125P7D:
https://www.hammfg.com/files/parts/pdf/290EX.pdf
Or, the slightly more expensive Classic Tone 40-18005:
http://www.classictone.net/40-18005.pdf
But, I've had good luck with Hammond, so far, I'll stick with theirs.
Either looks like a good choice. Keep in mind that you will likely have to modify the Stout chassis cutout a bit.
Hoffman sells this 6A/125V rotary:
https://el34world.com/charts/images/rotarydiagram2.jpg
Maybe, similar, or the same, that you used?
Same
And, where did you get those cool, black tube shields? :icon_biggrin:
My shields came from a decommissioned FAA radar system. You can still find them on eBay...
https://www.ebay.com/itm/4-IERC-TR6-6020-Vacuum-Tube-Shields-for-9-Pin-Miniature-12AT7-12AX7-Similar/353164660945?_trkparms=aid%3D1110009%26algo%3DSPLICE.COMPLISTINGS%26ao%3D1%26asc%3D20200818142652%26meid%3D7cfaadf7973a4f4e9f42f4969139aaef%26pid%3D101196%26rk%3D4%26rkt%3D12%26mehot%3Dnone%26sd%3D192625646289%26itm%3D353164660945%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3Ddefault%26brand%3DIerc&_trksid=p2047675.c101196.m2219
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Here's the final layout. The board is 3/8" shorter. Keep in mind that the board is optimized for 1/2W metal film resistors and Xicon "chicklet" caps. Mallory 150s will also fit but don't even try to use orange drops or some of those big booteek caps. If you have any questions just ask.
EDIT... removed attachment
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Thanks for all the work sluckey.
I'm ordering parts today.
I'll use 1/2W resistors everywhere except where your layout states 3W parts.
Glad I read your post about using the smaller Xicon's before ordering!
I'm taking a 4-day vacation a week from today; I should be able to get everything by then if I order now. So, I'll have plenty of time to work on this thing. :icon_biggrin:
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Everything's ordered. Most stuff, including the chassis, from Hoffman.
Going with the Classic Tone PT, OPT, and choke from TubeDepot worked out the cheapest after including shipping. Plus, I got the tubes, too.
As for a cabinet, I saw this thread, but it's three years old, so probably none left:
https://el34world.com/Forum/index.php?topic=21891.0
SILVERGUN, I might send you a message about the plans.
I was warned about this hobby. It's all out of control, now.
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Couldn't leave it alone! I've added one turret to the board to make it easy to add a voltage divider resistor in series with the GAIN pot just in case the amp has too much gain. I also added a note to your schematic. This is the last final change unless you spot an error. :laugh:
I highly recommend bookmarking the page on Hoffman's plexi6V6 build in this chassis. Lot of good building tips and pics that show a good way to do the wiring. Worth the read. Keep in mind that this is the prototype chassis and is very slightly different from his version 2 chassis. You will likely have to do a bit of chassis "tuning" to make your PT fit the precut hole.
https://el34world.com/projects/Plexi6V6.htm
I'm gonna remove the previous docs for this project so use this attachment and delete older files you may have.
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Thanks again sluckey. I've bookmarked that page. Nothing beats having clear hi-res pics.
Since I bought a blank board and a bunch of turrets I'm going to make one of these turret press tools for my drill press. Both Hoffman and TubeDepot have all the info needed.
I need to read up on gain in guitar amps. From what I understand, there are definite audible differences between preamp tube vs power tube gain. I don't know enough to really understand how overdriving the third triode before the PI will sound. So, thanks for the additional resistor option. I suspect the amp will want some tweaking here and there. But, it's a basic enough circuit (no trem or reverb) that changing caps and resistors - one at a time - should be relatively straightforward. :dontknow:
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I wanted to point out something about your anvil.jpg. It implies that the part of the tool with the hole in it will be mounted to the drill press table. That's wrong and if you try it you will quickly see why. Mount it as shown in this pic and load several turrets. Now you have to turn the board upside down to press them. I promise that some of the turrets will fall out. :icon_biggrin:
You want to mount the pointy part to the table and put the part with the hole into the chuck. Doing it this way makes it possible to put a lot of turrets on the board and press them all at the same time.
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I wanted to point out something about your anvil.jpg. It implies that the part of the tool with the hole in it will be mounted to the drill press table. That's wrong and if you try it you will quickly see why. Mount it as shown in this pic and load several turrets. Now you have to turn the board upside down to press them. I promise that some of the turrets will fall out. :icon_biggrin:
You want to mount the pointy part to the table and put the part with the hole into the chuck. Doing it this way makes it possible to put a lot of turrets on the board and press them all at the same time.
Done.
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You may find this interesting...
https://el34world.com/charts/toolhowto.htm
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Thanks sluckey, yes, I did watch Hoffman's videos on that page, and I went back and tapered the press (the part in the chuck with the hole) to accommodate closely spaced turrets.
I also found this cabinetmaker who has built Hoffman Stout cabinets before, and has very competitive pricing:
http://www.trmguitarcabs.com
By this time next week I should have the initial chassis wiring well under way! Just waiting on parts delivery.
Hey, thanks for taking an interest in my project sluckey. I haven't seen a Bassman mod exactly like mine, and if it does something well - either lots of "musical" gain, or the ability to take pedals at lower, clean volumes - I'll feel good.
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Got up early and knocked out the turret board.
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Your board looks great. Did you follow my instructions for drilling or did you use some other method?
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Your board looks great. Did you follow my instructions for drilling or did you use some other method?
Thanks sluckey! I Super 77'd your drill guide to the plastic protective covering on the board and used a 3/32" bit in the drill press. It lined up perfectly.
As Hoffman states in his video, it did not take much pressure to swage the turrets. The whole process went smooth.
But, it took a bit longer to enlarge the chassis for the giant PT. The 6L6 on the right is going to be close.
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Like this close:
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That's OK. People are not gonna believe what a monster amp lives in that tiny chassis! :icon_biggrin:
Your OT and choke will likely need new mounting holes drilled. It's worthwhile to also consider how the board will be placed and slightly adjust the board position, OT position, and choke position to avoid any conflicts. I attached a pic that shows Hoffman's choke and OT position. My board position is adjusted to avoid conflicts. Don't get in a hurry and you'll be fine.
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Don't get in a hurry and you'll be fine.
:thumbsup:
I suspect most problems on builds start by not following the above advise
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Thanks. Great advice. I'll temporarily put in the things that can't easily be moved, like the PT, pots and tube sockets. Then I can shift the board, choke and OPT around until nothing is cramped. Double check everything, and then drill.
Yep, it's going to be compact and look mild-mannered, but the weight alone will belie it's appearance. And, if I go with a really efficient speaker (like a 103db Eminence, etc.) look out. :icon_biggrin:
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mine started out as 2 x EL34 in P-P - ended up with 2 x 6550 in P-P. nice tones. that little box gets warm. it was a 2018 x-mas gift to my nephew.
--pete
https://www.dropbox.com/s/upuqmxwmwxk7ck0/20181107_223103.jpg?dl=0
https://www.dropbox.com/s/td5b497u434bdrn/Boilermaker_20181021_235128_sm.jpg?dl=0
https://www.dropbox.com/s/fhi3vczzc3r44rb/20181105_173813.jpg?dl=0
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thats Great Pete -
I'm going to be building this AA864 amp soon. i'm using a AMI jukebox chassis that i have. thanks dwinstonwood and Steve
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Thanks DummyLoad. I'll follow your placement of the OPT and choke.
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Hello sluckey, a question, please...
I'm building this project with one input (33k grid resistor, so "HI" only), and one speaker out (I bought a 3-way rotary switch and an OPT with 4-8-16 secondaries). A total of two 1/4" jacks.
Do I want two Switchcraft #12A's, two #11's, or one of each?
I'm thinking that two #11's are what I need - since there won't be another jack to ground when not in use.
Maybe I just answered my own question. Since both the input and speaker jacks will always be in use when the amp is being played, there's no need for #12A's.
Thanks.
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You want two Switchcraft #12A's. Connect the switch lug to the sleeve lug on both jacks. This puts a ground on the input jack tip when nothing is plugged in and this will keep the amp quiet. The speaker jack will also put a short across the OT when nothing is plugged in. The short is much better than an open which could damage the OT and output tubes/socket if eddie starts wailing away on the guitar with no speaker connected.
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Thanks!
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Taking my time, double checking things, reflowing solder joints, etc...
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Checkmarks work OK on a simple board. I prefer to use a highlighter. Works well for me, especially complicated boards.
(http://sluckeyamps.com/VAC15/04.jpg)
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Cool, I can see it's pretty much impossible to overlook a jumper using a highlighter.
I'm glad now that I didn't use a Fender chassis, and went with the Hoffman Stout that you suggested sluckey. It's probably about the best bargain available for a rigid, pre-punched chassis. It's like 2.5mm thick aluminum. It's easy to drill and cut, but there's no flex, even with all the heavy iron bolted to it.
I'll see if I can get the heater wiring done tonight.
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fwiw;
I've built on 3 of those chassis NO regrets!
did you print out the docs BIG?, I usually get something in the 14X17 size. I use yellow pre-solder layout, then red for soldered, then blue for double-checking/verifying.
I'm an artist so I have to :icon_biggrin:
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No, but I'm going to print out the layout at 17" since I hate squinting! :icon_biggrin:
I like the idea of using different colors. I had to double check every turret to be sure I had a good flow of solder.
shooter, any suggestions for a speaker config? I'll have a 4, 8, and 16 Ohm selector. Two 10's, three 10's, two 12's? six 8's? :laugh: Thanks.
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I'm reminded of a chevy vega with a 327 engine. :icon_biggrin:
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chevy vega
when my folks made it to upper poverty Dad bought Mom that version!
3 yrs latter I was going back to Base and my car wouldn't start, Mom loaned me the vega, coming back home the following weekend I got T-Boned by a drunk that blew through a stop at 70mph
I offered my '70 Camero but Mom wouldn't take it. She ended up with a well done '65 Nova
Ok, Speakers;
roll as many as you can find, begging n borrowing, then decide on how many and with type
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upper poverty
:laugh:
nice project dw!
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Thanks bmccowan!
I got busy with other stuff, so progress has been slow. I have a tough time wiring up heaters. I usually have to start over at least once. But, it's done!
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OK, I'm stumped. A total brain lapse. :w2:
I'm using the 120vac power light that Hoffman sells.
His wiring instructions say:
"One wire goes to the neutral/white power cord wire.
The other wire goes to the hot/black power cord wire after it comes out of your main power switch."
I can't figure this out. I thought the white neutral wire from my PT went to the neutral pin on my power cord socket. I just can't picture how to wire it up. :dontknow:
Thanks!
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Connect the PT white to the IEC neutral lug. Then connect one lug of the lamp to the neutral lug. Now connect the PT black wire to the switch. Also connect the other lamp lug to the switch.
IOW, the lamp is connected parallel to the PT primary.
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IOW, the lamp is connected parallel to the PT primary.
Ah! Now I get it. Thank you sluckey. :worthy1: I couldn't picture it.
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The lamp is a neon bulb, not a LED.
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The lamp is a neon bulb, not a LED.
:think1:
Wow, it took me most of the day to get this far, and I haven't even started on the jacks and pots. Hopefully, I can get that stuff done tomorrow.
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I haven't even started on
See reply #56-57 :icon_biggrin:
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Well, one more quick question...
I'm thinking that even though I'm using a speaker Ohm selector switch I still want to connect the NFB to the speaker jack tip, yes? That way it will stay connected regardless of the selector setting (4, 8, or 16).
But, I could be all wrong about that. :icon_biggrin:
Thanks!
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I strongly suggest using four 3/4" fender washers on that PT. If can't find any, then four pennies would be cheaper. :icon_biggrin:
The NFB wire should connect to the OT 4Ω tap at the Impedance Selector. You want the NFB to remain constant.
EDIT... I don't see any holes drilled for mounting the board. That should have been done at the same time you drilled holes for the OT and choke. The more stuff you put in that chassis, the more difficult it will become to accurately drill the board mounting holes. It's already become quite a challenge.
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Thanks sluckey. I was keeping it in mind, and I knew the relocation of the choke would be an issue. I just needed to move one of the mounting bolts down 1" to clear it.
What I didn't take into account was the large size of the terminal strip. So, I modified it, making it about 1/3 of its original size. It's just as strong, but doesn't interfere with anything now.
It's definitely a cramped chassis, but there's enough room; nothing is contacting other parts.
The good thing is that most of the wire runs can be very short. In fact, I see in Hoffman's 6V6 Plexi build that he actually connected the grid resistors directly from the jack to the tube socket. But, I have 20' of RG174, so I can still use lots of shielding.
This photo, in particular, is serving as a good guide for wire choice, etc.:
https://el34world.com/projects/images/Img_9095.jpg
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That relocated screw sure makes me nervous. The screw head is very close to the highest voltage in the amp. And what about the standoff under the board? I bet the standoff is even closer to that turret. If either the screw or standoff touch that turret it's bye bye diode. Puts the PT at danger too.
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That's a good point!
I'll use nylon standoffs and turn the screw head down. Or, I can drill another hole in the board, and move it. :icon_biggrin:
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I think this is right. :dontknow:
I've never wired one of these before...
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This is a good stopping point for today. I still need to add the power and preamp grounding point connections. It's a lot easier for me to deal with that after all of the power wires are routed first.
Tomorrow I'll mount the pots and input jack on a thin board and wire all of that up, attach the ground buss, small parts, etc. I know I can't do that down inside the chassis. Then, install that complete assembly back in the chassis.
I'm following Hoffman's 6V6 Plexi guide, so I'll add the parts to the board after that. Then, I'll add the wires from the board to the pots, just like his guide.
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Tomorrow I'll mount the pots and input jack on a thin board and wire all of that up, attach the ground buss, small parts, etc. I know I can't do that down inside the chassis. Then, install that complete assembly back in the chassis.
Why? You already have the perfect rig to do that. Just install pots, jacks, etc, on the outside of the chassis. Wire it up. Then flip to the inside. Everything will fit perfectly.
(http://sluckeyamps.com/november/nov_02.jpg)
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Well, I went with your method sluckey. It was easy. In fact, the weight of the chassis kept things from moving around on the desk. :icon_biggrin: Without your layout/wiring diagram this would have been a frustrating wiring project. So, thanks again.
I used shielded cable for the three connections going directly to grids. These seemed like the most critical places to use RG174. :dontknow:
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like the wire colors :thumbsup:
shielded wires are great, just make sure no shorts to shields
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It is easy. But you overlooked one small but very important detail. You wired them upside down. Now when you flip the assembly over and mount on the inside, the pot lugs will be pointing away from the open chassis. It will be nearly impossible to get to the lugs later. I highly recommend undoing, rotate everything 180 degrees, and redo.
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Hmm - this will likely lead to some musing. Sorry, but what the heck, its only a few wires, eh?
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It's fixed. Took a bit over 30 mins.
I got confused by your layout sluckey, and read it wrong. Something about the way I was picturing the folds in the chassis made it look like the tabs were pointing into the chassis. :w2:
I can't see an error in my wiring - it matches the layout - but, I probably have something wrong! :icon_biggrin:
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I got confused by your layout sluckey, and read it wrong. Something about the way I was picturing the folds in the chassis made it look like the tabs were pointing into the chassis. :w2:
Much better. I can see where it could be confusing. That's the first drawing I've done with the pots on the tube side. All my other layouts have the pots at the top of the page. Less confusing.
BTW, the switch on your volume pot is pull on/push off, right?
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BTW, the switch on your volume pot is pull on/push off, right?
Thanks sluckey.
Yes, it's the one Hoffman sells. Here's his description:
"CTS amp pots with SPST pull switch
Used for master volumes, bright switches and other switching on Fenders.
The switch is on when the pot shaft is pulled out.
Standard 3/8 inch mounting hole."
It's been a mind bender for me. I'm building a Fender amp in a Marshall-style chassis. It's like upside-down and mirror image (if you think about the traditional Marshall control order). Funny, I never thought about that before, but early Marshalls are that way. Maybe so they didn't look anything like Fender Bassman copies?
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Well, somehow I failed to order 100 Ohm resistors (must have been thinking 100K). So, the NFB circuit isn't complete. This also grounds the LTP. All the other parts are in.
Next step is to connect up the two ground points. Speaker jack, input/pot buss, and preamp ground at point "b" in the pic. Everything else, including the bias as well as the 220K 3W cap resistor at point "a".
Then, I'll focus on the wires from the pots.
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Looking good!
Well, somehow I failed to order 100 Ohm resistors (must have been thinking 100K). So, the NFB circuit isn't complete. This also grounds the LTP. All the other parts are in.
If you have a 47Ω use it instead of the 100Ω and move the NFB wire to the 8Ω OT tap.
Next step is to connect up the two ground points. Speaker jack, input/pot buss, and preamp ground at point "b" in the pic. Everything else, including the bias as well as the 220K 3W cap resistor at point "a".
Move point b (preamp ground) to be very near the input jack. The only thing that will connect to it is the pot buss and the preamp ground. Everything else connects to point a (power ground).
DO NOT CONNECT SPEAKER JACK TO PREAMP GROUND!
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Thanks sluckey. This is why I post progress pics. :icon_biggrin: In hopes that you'll catch my goofs. Truth is, grounding is still a confusing subject to me (I read up on it, and forget what I've read).
So, just so I'm 100% clear, the OT secondary has high current (and, if it shorted internally, high voltage), and therefore the speaker jack should be grounded along with the power and B+ grounds and away from the preamp, yes?
As for installing the parts, this time I didn't have any trouble soldering to the turrets. By following Hoffman's suggestion: making an initial solder, and then going back and reflowing a bit more solder, I got nice, shiny domes on top of all the turrets - instead of having solder run out the bottom of the turrets.
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So, just so I'm 100% clear, the OT secondary has high current (and, if it shorted internally, high voltage), and therefore the speaker jack should be grounded along with the power and B+ grounds and away from the preamp, yes?
I'll give you 90% for that answer. Just forget about the voltage. The OT secondary has low voltage but high current. That high current develops a pretty strong electromagnetic field around the wires. This strong field is capable of inducing signal into the low voltage high impedance sensitive preamp stuff. This can cause all sorts of problems for you to chase down later on. Just connect the speaker ground wire to the power ground and don't worry.
Also keep any of the OT secondary wires as far away from the preamp circuit as possible for the same reason.
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100 Ohms from home with this project. :BangHead: Sorry. :icon_biggrin:
Well, a few other things... washers for the PT, tighten all bolts, trace everything with a highlighter, etc. I did measure each and every resistor before soldering (I can't read color bands).
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100 Ohms from home with this project. :BangHead: Sorry. :icon_biggrin:
You can replace that 100Ω with a wire jumper and start playing music. Doing so simply means there will be no NFB until you can put a 100Ω in the amp. :guitar1
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OK...
I'm always nervous about this. Let's see: [After adding the jumper] With no tubes in I'll flip the switch on the dim bulb tester. If nothing loud, flame-like or smelly happens I'll turn it off and plug it into the wall. Then, I'll hook up the multimeter to to measure ACV on the heater pins. Repeat the process to measure pins 3 to ground, and pins 4 to ground on the Octal sockets. Do the same for pins 1 & 6 on the 9-pin sockets. (I've already measured the cathode pins to ground and I have the correct resistance measurements for each socket.)
What I don't know is the negative bias voltage measurement I should want to see on pins 5 with the bias pot turned to maximum negative voltage. I want to be sure it's low enough before installing tubes.
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What I don't know is the negative bias voltage measurement I should want to see on pins 5 with the bias pot turned to maximum negative voltage. I want to be sure it's low enough before installing tubes.
Without the lamp limiter, turn the bias pot for maximum negative voltage on pin 5 of both 6L6s. Hopefully you will have at least -50V. If so, then plug the tubes in.
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Thanks!
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I'm going slow with this.
7.0VAC on the heaters
6L6 #1 pin 3 - 490.5 VDC
6L6 #2 pin 3 - 492.7 VDC
6L6 #1 pin 4 - 493 VDC
6L6 #2 pin 4 - 493.2 VDC
However - with no tubes in - I'm measuring in the 480's on the 9-pin plate sockets (1&6). Is something wrong there? I don't want to proceed if that's bad. :dontknow:
Thanks!
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However, I am getting a range from -47.85VDC to -36.04VDC on both #5 pins.
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However, I am getting a range from -47.85VDC to -36.04VDC on both #5 pins.
May need to tweak that range a bit. But for now leave it set to -47.85 and plug in the tubes and speaker.
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Cool. I'm eating lunch first - mainly to clear my brain and re-focus, and then on to installing tubes...
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It will only take me about 3 minutes to swap the 470 with a 1K. It's a quick fix (assuming that's the best resistor in the bias circuit to tweak).
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With the original 470 Ohm bias resistor still in the circuit I'm measuring across the 1 Ohm resistors and then, the plates:
Tube #1 (outer tube) - 0.049VDC (49ma)
Plate - 463VDC
22.687W - 75%
Tube #2 - 0.047VDC (47ma)
Plate - 463.2VDC
21.77W - 73%
I guess my math is right? I multiplied the plate voltage by the milliamps.
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I'm wondering if my 12_7's are running too high on the plates: :dontknow:
1st 12AX7: pin 1 - 282.9, pin 6 - 281.3
2nd 12AX7: pin 1 - 286.2
12AY7 PI: pin 6 - 261.3, pin 1 - 253.2
At any rate... it sure sounds great. On "3" with the gain on "5" and my guitar volume 1/2 way up it's loud in my room. I'm playing it through the 6G3 75 watt speaker.
Oh, and at the lower volume (I haven't cranked it) it's extremely quiet. Wow. I need to chill for a bit and just enjoy the result.
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It will only take me about 3 minutes to swap the 470 with a 1K. It's a quick fix (assuming that's the best resistor in the bias circuit to tweak).
No. That would just decrease the bias voltage and you need to increase it to about -55V when the pot is at one end. Measure the voltage at the junction of the diode and the 10µF cap. What have you? We'll be messing with that 15K and/or 27K that's connected to the bias pot to get the bias voltage a bit higher.
As for the high voltages on the little tubes, you'll need to increase the size of the 1K and 4.7K dropping resistors on the B+ rail. But first, measure the voltage at nodes A, B, C, and D. What have you?
BTW, Good looking amp.
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Thanks sluckey. Yeah, I went back and re-read Hoffman's bias instructions and I see know that raising the 470 would make the circuit less negative. :think1:
So, should I take these measurements with the tubes in or out?
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The diode/10uF cap junction measures -69.3VDC. That's without the tubes in. I'm guessing it shouldn't make a difference.
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With the tubes in:
cap can A - 468.4VDC
cap can B - 468.1VDC
directly in front of/before first 22uF cap - 460.3VDC
directly in front of/before second 22uF cap - 440.6VDC
I might not have measured at the right points.
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The schematic shows 410 at the third node and 380 at the fourth. So, I'm roughly 50-60 volts high on these nodes. Nodes one and two are 422 and 420, respectively, so about the same amount high.
Also, the schematic shows 235 on the first two triodes, 240 on the third, and 220 & 205 on the PI.
It seems like all of these current production transformer manufacturers attempt to make exact replicas of the originals, which is fine, but they should have - IMO - engineered today's wall voltages into their products. I know it would drive the prices up, but it would be nice to have both "vintage correct" and "current-day" secondaries. With 10% to 15% lower secondary voltages you could build vintage amps with vintage-correct plate voltages.
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I've been trying to work on this, but I can only get so far.
The three preamp triodes all have 1500K cathode resistors. If I divide each of the schematic's cathode voltages by 1500 and add it all up I get a total of 3.9ma of current draw for the preamp.
I want to drop about 50v. So, that indicates an extra 12.8K of resistance.
The problem is I want to drop the PI voltage, too. And, I don't know how to calculate the 12AT7 current draw from the LTP in the schematic. Plus, that tube comes before the preamp, so I really need to drop that voltage first before I can calculate the preamp resistance I need.
There's always the blind stab in the dark method. I could replace the 1K with the 4.7K, and replace that with a 10K and see where that gets me.
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Well, if the voltage drop across the 1K resistor is 8V (468 to 460), then the PI is drawing 8ma. So, 50/.008=6250.
I think I have a 6.8K 3W.
Then, I only need to drop the preamp voltages by 30V. So, 30/.0039=7.69K. I think I also have an 8.2K 3W.
That may be a better place to start. :dontknow:
The JJ 6L6GC's can take up to 500V on the plates, so 468 seems ok. But, the screens are rated at 450V. Could I just increase the screen resistor values? It can't be that simple.
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Quit thinking of this amp as an AA864. What you really have is an AB763 with an extra gain stage. More like a Super Reverb with the iron you have. Don't worry about the screens. They are fine.
Now, for the fun tweaking phase...
Here's a plan. Tackle the bias voltage first. Pull the 6L6s until you have approx -55V on pin 5 with the bias pot at max. First, change the 15K to 10K. Recheck pin 5. If you still don't have approx -55v then also change the 27k to 33K. Should be getting closer to the target. Continue tweaking these two resistors until you are satisfied with the pin 5 voltage. Notice that one resistor increases the bias voltage as the resistance goes down and the other resistor increases the voltage as the resistance goes up. Should be pretty easy to hit the target. Now plug in the 6L6s and set the bias to 70% or even 60%, whatever sounds good to you.
Second... Set the PI voltage for what you want. Increasing the 1K will decrease the PI voltage. Once you are happy with the PI voltage, move on to step three.
Third... Recheck the preamp voltages. You just may be where you want to be now that you changed the PI voltage. (Changing the PI voltage also changes the preamp voltages). But if still too high for you, increase the size of the 4.7K until you are happy.
Done!
Don't get hung up on some magic voltage you have seen on some schematic. There's really no right or wrong here. Just adjust things until you like the way the amp sounds. Make it your own!
Oh, almost forgot... When you finally replace that jumper with a 100Ω resistor your NFB loop will be working. There's a 50/50 chance that the amp will squeal or howl. If so, just swap the two white wires that connect to pins 1 of the 6L6 sockets, but do the actual swap on the board. Very easy to neatly swap on the board since they are right next to each other.
BTW, how does the extra gain stage sound? This could be a place to try some different tubes for different gain levels. Try 12AT7, or 5751, or 12AY7, or even a 12AU7. All have different gains and you may find you have a preference.
Enjoy.
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> I don't know how to calculate the 12AT7 current draw from the LTP in the schematic.
Ohms Law is the law. Obey it. Use it.
There are enough numbers on the factory plan to compute essentially "all" currents. They don't quite add-up right, because the draftsman didn't know we would be checking. But they agree to better than 20% which is golden in most tubewerk. Yours will be different again, but you can do the same figuring.
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> I don't know how to calculate the 12AT7 current draw from the LTP in the schematic.
Ohms Law is the law. Obey it. Use it.
There are enough numbers on the factory plan to compute essentially "all" currents. They don't quite add-up right, because the draftsman didn't know we would be checking. But they agree to better than 20% which is golden in most tubewerk. Yours will be different again, but you can do the same figuring.
Thanks PRR! Up in post #117 I actually did use the voltage drop across the 1K resistor (8V) to calculate 8ma current draw at that stage. But, you're right. I'm not always approaching this amp stuff from the right vantage point.
People here have told me before that getting too zoomed in on one issue is a problem. It wasn't until I stepped away for awhile that I started thinking about Ohm's Law. Then a lightbulb went off. I was obsessing over differences in wall voltage and current-production TX's, instead. :icon_biggrin:
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OK, sluckey, here's where I'm at:
I installed a 6.8K resistor in place of the 15K. This gives me a maximum of -56.07VDC on pins 5 with no tubes in. I didn't have a 10K or 8.2K on hand. But... I know I like the sound of cooler running power tubes - 60-ish% - so I'm thinking I'm fine with the 6.8K.
With tubes in:
I'm now at 477.4VDC on the 6L6GC plates. Still within spec.
I replaced the 1K B+ dropping resistor with an 8.2K. I left the 4.7K in place.
Here are my 9-pin voltages. Much better:
V3 (PI) pin 6 - 238VDC; pin 1 - 228VDC
V2 (gain) pin 1 - 257.9VDC
V1 (pre) pin 6 - 253.5VDC; pin 1 - 254.3VDC
I didn't have a 6.8K. I think I'm ok for now. I'll probably order some more resistors and I might try a 6.8K instead of the 4.7K. That would drop the first three triodes down a bit more.
OK. I guess the next step is to bias the power tubes. It looks like I want about 38ma to be in the 60% range, or 44ma for 70%.
Onward.
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I installed a 6.8K resistor in place of the 15K. This gives me a maximum of -56.07VDC on pins 5 with no tubes in. I didn't have a 10K or 8.2K on hand. But... I know I like the sound of cooler running power tubes - 60-ish% - so I'm thinking I'm fine with the 6.8K.
I think you're good to go on the bias. But before you put the tubes back in rotate the bias pot to the other extreme. What is the voltage on pin 5 now?
Then return the bias pot to -56v, plug in the tubes, and set the bias where you want it.
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sluckey, I forgot to measure the sweep with the tubes out.
But, I'm there!
If you picture the bias pot as a clock face it's turned to between 11 and noon, just below halfway.
That gives me 38ma on each grid.
New plate measurements are 468.2VDC and 468.1VDC
That's right on 60%. 17.79 watts from the outer 6L6 and 17.78 watts on the inner 6L6. Is that good enough? :icon_biggrin:
Thanks so much sluckey for your interest and help with this project! And, thanks to everyone else here who helped me through this. This forum is great. :worthy1:
I'm basically the monkey who can soldier in resistors and use a multimeter, but I really don't know how to make electricity perform the acts that I want it to.
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So, what do you think about the extra gain stage? Is it controllable?
That gives me 38ma on each grid.
Surely you mean cathodes?
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Yes, I meant 38 millivolts across the 1 Ohm cathode resistors! :laugh: The adrenaline was up when I posted.
As for the gain control. I haven't cranked things up too far yet (Sunday morning is not the best time where I live). But, with the volume on 4 and the gain on 7 it's not unlike having a built-in overdrive. In fact, I'm getting sounds that are very similar to the BYOC TS-808 clone pedal kit I built. I need to play with it more to hear all the various settings. Once I get a good idea of the gain range I'll decide whether or not it needs a resistor instead of the jumper. Also, I installed the 100 Ohm NFB resistor, so that's hooked up, too.
With gain down around 2-3 and the volume up it's nice and clean. This amp is definitely not too bright. I'm setting treble at 6 and bass at 3 with bright off and it's very balanced and full sounding. But, it can be made bright with the switch pulled out and using the bridge PU. The push/pull bright switch is like the normal and bright inputs on my 6G3. But, I obviously don't have to unplug and plug back in to change it. Thanks for that addition. It's convenient.
So far, I find having the gain around 4-5 and volume around 3-4 is nice. A hint of overdrive that's controllable by string attack.
I'll report more after playing some more. Thanks!
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BTW, how does the extra gain stage sound? This could be a place to try some different tubes for different gain levels. Try 12AT7, or 5751, or 12AY7, or even a 12AU7. All have different gains and you may find you have a preference.
After I play it more I may want to try different gain stage tubes. I was just reading the JJ 5751 description:
https://www.jj-electronic.com/en/5751
At $13.95 it's a cheap experiment to see how less gain/more headroom would sound. Though, I don't really need maximum clean headroom volume from this amp. Someone else might, so it's definitely a tweakable build.
It's a versatile amp from what I can tell so far. I'm only speaking of bedroom volumes as of now, but with the gain at 2 and the volume on 4 it has a very clear, smooth tone. Really nice, not hard or harsh, but plenty of treble. As I mentioned, reverse those two settings (say volume on 2-3 and gain on 4-5) and even at that low volume there's some overdrive in the sound.
Initial impressions are that it's more and better than I was expecting. Though, it could all fall apart and sound horrible when really cranked. I guess a lot of that will depend on the speaker(s).
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gain at 2 and the volume on 4
I'm home alone; both on 7, guitar 6-10, you play longer when it's louder :icon_biggrin:
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I'll be in that situation tomorrow, when the neighbors aren't home. :icon_biggrin:
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A morning of tweaks and fiddling.
One last change to the B+ dropping resistor: preamp plates range from 245VDC to 249VDC. If there is an audible difference in 10 to 15 volts either way, my ears aren't going to hear it. Done.
I added the tube clamps on the octal bases, tubes are nice and firmly in place now.
I'm not sure I'm in love with the push/pull volume pot. It does nothing until around 2 or so, and if I wiggle it I get some scratchy noise. I double checked all solder joints, and they're shiny, solid and thorough. I think the mechanical design of this pot has some play/wiggle in it. I might just go to an audio taper and ditch the bright feature. My Tele and my ears don't really need it anyway. I'd like to have the most precise control below 50%, especially for playing at night.
I've been poking around in my 6G3, too. I added two more shielded wires, bent some signal and power wires around, and redid both grounding points. This amp is now noticeably quieter than it's ever been. I'm not 100% sure exactly what I did, but I'm not messing with it anymore.
Finally, I swapped the 100K grid resistor in my AC4 for a 56K (sluckey's AC15 schematic shows a 33K, but I didn't have one). It sounds better/brighter. Probably like going from a low input to a medium-high input. I think it sort of woke up the EF86 from being a bit drowsy. I also replaced the tan grill cloth with Vox diamond cloth, so it now resembles a Vox.
I built these three tube guitar amps (my first ever) over the last seven months with the help and guidance of you all here. All three amps and great, each with it's own unique character.
Thanks!
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One last change to the B+ dropping resistor
So what values do you have for the two B+ dropping resistors?
And what voltages do you measure for nodes A, B, C, and D?
And what is your input line AC voltage?
And which OT and PT are you using?
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Thanks sluckey, here's the data:
1st dropping resistor - 10K 3W
2nd dropping resistor - 10K 3W
Node A - 471.8VDC
Node B - 471.1VDC
Node C - 395.4VDC
Node D - 359.2VDC
AC Line In - 123.2VAC
PT - Classic Tone 40-18005 (330-0-330 @ 250ma; 6.3V @ 4A; 50V bias)
OPT - Classic Tone 40-18006 (4K Ohms; 4,8,16 secondaries)
Choke - Classic Tone 40-18003 (4H @ 90ma; D.C.R. - 105 Ohms)
David
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Here's the final as built drawings. They include your changes and voltage readings.
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That is really generous of you sluckey. Not just the documents, but your oversight/corrections, guidance, and shared knowledge in this project are much appreciated. I learned a lot with this build. And, it was great fun.
I have an amp that looks good, sounds great, and can go from "AB763 bright & clean" to dirty with no pedals.
:occasion14: :headbang:
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FYI, the AB763 Tremolux and Vibrolux (same amp but different speakers) use 10Ks for B+ droppers. But B+ is a bit lower because of smaller PT and tube rectifier.
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Cool. That's good to know that my choices are not unheard of values in Fender amps. :icon_biggrin:
[With the 22uF caps that makes two low pass filters with a cutoff of 0.7237984945 Hz. Merlin's website says we want the cutoff below 1Hz if possible for B+ filtering, so in that respect the 10K's look to be a good value.)
Not to bemoan current production transformers again, but the schematic indicates a PT with 305-0-305 secondaries. The one I bought is sold as a 125P7D replacement, yet it's 330-0-330.
But, all of my plate voltages are now good, so I'm not really complaining! :icon_biggrin:
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I spent three hours at a local recording studio this morning with the amp. I played my Tele, a Gibson Les Paul, and even a Martin acoustic with a clip-on Fishman. :icon_biggrin:
With the gain on 10 and the volume around 6 it is very loud through that Marshall cabinet, and it's a big room. To my ears it really mates well with the Fender. I have a 5751 in the gain location now. It's crunchy but not what you'd call real distortion (I didn't dime the volume). I need to go back and try a 12AX7 next weekend. Though, it sounds great clean with a bit of reverb. And, I'm glad I installed the speaker selector switch; the cabinet was 16 Ohms.
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Done.
I ending up using grounded RG174 for both the send and return leads to the gain pot. I'm using a 5751 for V1 (something tubenit suggested in another thread). And, I repositioned the OPT primaries up away from the heater wires.
It's now the quietest it's ever been. There is a low hum when you lean close to the amp, but not any more than my other amps (I might check the PT bolts for tightness).
So, thanks to all the help here - in fact, sluckey more or less built this amp remotely - I totally consider this project a success.
And, it's relatively compact: 24" high, 18" wide, and 10" deep with a 12" speaker.
Since it didn't cost me anything (used the laser engraver at work) I made a back panel, too.
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Sorry to keep resurrecting this thread. :icon_biggrin:
I took the amp to a studio again tonight to wring it out. I'm really happy with the way it sounds. Adding the mid control was worth the challenge it caused me - a simple mod for most here. I can really get the most out of a range of different guitars now.
I couldn't be happier with it.