Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: glassesnvests on July 02, 2014, 05:49:14 pm
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So I endeavored to take on a schematic I found online of a JTM45/AA864 amp, and I've run into an issue.
Initially I set out to add in a VVR and an effects loop (building for a friend, that's what he wanted), so I incorporated Hall Amplification's Fixed bias VVR and put in tubenit's tube FX into the place between the preamp and LTPI.
So my problem is that when I fire up the amp, it doesn't get very loud. At all. I can have the volume level (on both channel 1 and 2) and the volume doesn't get near what a 5 watt amp can at 12 o'clock. And it distorts a fair amount. No overt hum.
So I've resorted to asking you guys for insight. I'll get pin voltages posted asap. A link to my photo album is below. The wiring job isn't great, I know.
I have completely disconnected the FX loop for the sake of trouble shooting.
http://s840.photobucket.com/user/Adam_McGoon/library/JTM45-AA864 (http://s840.photobucket.com/user/Adam_McGoon/library/JTM45-AA864)
Original schem and the .sch file I made is attached. By the way, the FX switching in real life follows tubenit's design, just not represented in the schematic.
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Pin voltages below.
B+ is 450v before and after inductor.
1st filter cap: 450v
2nd cap 20uf: 439v
3rd cap 20uf: 395v
4th cap 16uf: 335v
Thanks for the help!
Oh, yeah. Transformers are Classictones:
PT: 4018005
http://triodestore.com/bahecoprbapo.html (http://triodestore.com/bahecoprbapo.html)
OT: 4018001
http://triodestore.com/surebareupho.html (http://triodestore.com/surebareupho.html)
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Q1) With the FX disconnected and out of the circuit, does it still have the volume problem?
Q2) Does it have the Dana Hall VVR in it? I can not tell that it does from the schematic and photos? And did you try it with the VVR
disconnected if it is currently in place?
Q3) Can you post plate and cathode voltages, please?
Q4) Have you checked tubes to make sure they are good?
Q5) Do you have a known good speaker, guitar & guitar cord?
Q6) Is there a chance that the low volume and distorted tone was with an effects in the effects loop AND that you plugged it in
wrong (meaning backwards)? I say this because I built an amp for someone who made the same low volume distortion
comment and by switching how his delay pedal was plugged in ................... instantly resolved the issue and the amp
sounded beautiful.
Q7) Have you disconnected the NFB? Sometimes it doesn't squeal and instead will distort and have low volume.
With respect, Tubenit
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1) Yes, the FX is 100% out, and it still have low volume.
2) The VVR is nestled in the corner by the AC inlet. on the back of the amp. I have tried it with/without the VVR, and it persists. At the VVR at 'full', the B+ tracks the same pre and post VVR, no loss, and when I turn it down the voltage tracks down as it should. the powerFET is thermally sound and isolated.
3) See below.
4) Tubes are good. Power tubes are GT's that I can plug into my Dr. Z Galaxie and sound great.
5) Guitar, speaker (8 ohm 50w blue dog), and cord all tested with a homebrew AC15.
6) With no pedals plugged in, pedals plugged in, and FX completely out, all act the same.
7) I have indeed disconnected the NFB and swapped OT wires to check for that problem. The schem I found has a slightly variable one ( a 10k in series with an 820ohm)
Thanks!
| Pin # | | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| 12ax7 | V1 | 207 | 3mv | 2.8 | 6.3 | 6.3 | 183 | 2mv | 2.7 | 6.3 |
| 12ax7 | V2 | 128 | 2mv | 1.8 | 6.3 | 6.3 | 128 | 2mv | 1.86 | 6.3 |
| 12ax7 | V3 | 174 | 2mv | 1.34 | 6.3 | 6.3 | 337 | 174 | 176 | 6.3 |
| 12ay7 | V4 | 432 | 0.5 | 13 | 6.3 | 6.3 | 433 | 0.5 | 13 | 6.3 |
| 6L6 | V5 | -51 | 6.3 | 450 | 450 | -51 | 450 | 6.3 | gnd | xx |
| 6L6 | V6 | -51 | 6.3 | 450 | 450 | -51 | 450 | 6.3 | gnd | xx |
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The schematic you show in the first post shows +95V on both cathodes (pins 3 and 8) of one tube.
I do not see any such thing in your voltage-chart.
I wonder if the 470r has gone to ground instead of to the 22k.
Maybe I missed a mod.
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I can see anything wrong with schematic or the posted voltages?
Good news is that you're obviously knowledgeable about these amps and have already eliminated numerous issues, so I am confident that you will eventually figure it out. I would love to hear what the eventual solution is when you find it.
The only other thing that I can think of is a bad solder joint/lead dress. Or maybe an oscillation issue which can give distortion and low volume?
I will presume you already "chopsticked" the beejeebers out of it.
With respect, Tubenit
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Thanks, the both of you!!
Especially PRR, for getting my attention to the 470/22k at the PI. A very stupid mistake on my part. Instead of 470, I had 470K in that spot. Now it fires up and sings! Man it sounds great.
Thanks so much!! Now to get the Loop and VVR back in to test those...
Thanks again!
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PRR is a genius and then some! :thumbsup:
Glad you got it working! Hope you will enjoy the amp and have great fun with it.
Thank you for taking the time to let us know the solution, it is always appreciated when someone does that.
With respect, Tubenit
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Thank you for taking the time to let us know the solution, it is always appreciated when someone does that.
Yes!
Brad :icon_biggrin:
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> stupid mistake ... Instead of 470, I had 470K
That "can" be a smart thing. Up-ing the 470 resistor will "lame" the driver so it can not smack the output stage to MAX output, but some lower value.
Of course 1,000 X larger would be "low output" for-sure.
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tubenit, your effects loop is fantastic in this thing! Really adds another dimension to the feel of the amp.
Quick question, though, I get a very very loud pop when switching the FX off and on. Is there any way to ablate this? Maybe a bleeder resistor acrost' somewhere?
Thanks,
GNV
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I get a very very loud pop when switching the FX off and on. Is there any way to ablate this? Maybe a bleeder resistor acrost' somewhere?
I don't remember my D'Lator popping when switching???
And in my two current amps (Tweed BluezMeister & D'Mars ODS), the FX is always on (no switch).
Are you switching as shown in this schematic? Maybe Sluckey will weigh in and have an idea for you regarding the popping.
With respect, Tubenit