5E7 Problem and need help - ratty distortion at all volumes (Other post disappeared)

[telesupro]

Hello Everyone,

I’m a new guy here, just joined. I’ve been perusing for a long time though. I posted a thread earlier today but it seems to have disappeared, so please forgive me if it shows back up and this is a repost.

I really need some help on a new 5E7 type build that has an issue I can’t solve. Any help, advice, or direction is greatly appreciated! I’m still fairly green on things, but have built several tweed type amps without problem; that all changed last week. Here’s the info I have:

The Problem:
It passes signal, it works, but not correctly and sounds like doodie. It has ratty distortion at all volumes, even very very low volumes. It’s VERY GATED sounding - similar to how an Octavia sounds (without the actual octave in this case though) where the notes are gated. It’s almost the reverse of that though in that if I pick really soft, it will play but still has the terrible distortion. If I pick hard, the note(s) gate out, cut off. I can pick softly again and it will return to the ratty sound. It won’t really get loud because even if I turn it up I have to pick super soft or it clips. It sounds like I’m playing through a badly broken speaker but worse. I kind of thought it might be leaky coupling caps, but I have no idea at this point. I’m drained for ideas on where to look.

**One thing to note is that when I have the amp on and in standby and then flip the standby switch to full “on”, the amp sounds wonderful, big, and clean for about 2 seconds and then goes into what I described above.

Things I have tried: the speakers are good; they are in another functioning amp and have been checked and rechecked. I don’t think it’s the tubes; I’ve tried several different sets with the same result. I don’t think it’s the filtering/filter caps because I added more filtering just to check and it sounded the same (I removed the added filtering after checking). I don’t think it’s reversed output transformer leads that cause oscillation because I removed the negative feedback and have done all my testing with it disabled – same result. The cord has been checked, the underboard wiring has been checked, the resistor values have been checked (some read half of what they are, but as I understand it that is the way it is on some of them, ie 68k input resistors). All this being said, I know all too well I can be wrong (a lot), even on a recheck. So anything you guys suggest will be taken and acted on.

The Amp / Components:
It's a 5E7 with a bit larger transformers: Power Transformer: Magnetic Components Classictone 18073, 325-0-325, 650V centertap, 50V bias, 5V@3amp, 6.3V@5amp. Output: Allen Amps TO40MT, 4000 ohm primary to 4 or 8ohm. I wired the power tube sockets like later Fenders with a 1.5k from pin 1 to pin 5, and a 470ohm from pin 4 to pin 6. The NFB is connected to a Presence pot that has a switch when turned all the way down to defeat the NFB. I also moved the output transformer centertap upstream to the other side of the choke (Allen Amps TRCH 125C1A style, 3H, 100 ohms) between the plate and the screens; it’s connected to the bottom of the 1st filter cap.

Voltage readings:
All measurements are DC unless otherwise noted, all measurements take with amp on/tubes in/no signal/knobs at 0, all filament heaters read 2.9 to 3.0VAC, the voltage at the wall is 122VAC, I’m using a 12AY7 in V1 and have an old GE 5U4GB rectifier.


V1
pin1 152
pin3 2.44
pin6 159
pin8 2.44

V2
pin1 209
pin2 1.54
pin6 319
pin7 209
pin8 212

V3
pin1 243,
pin3 1.72
pin6 295
pin7 20.9
pin8 64.8

6L6GC
pin 3, read the same on both tubes 422
pin 4 , read the same on both tubes 422
pin 5, read the same on both tubes -39.4
pin 6, read the same on both tubes 423

5U4GB
Pin 2&8 to ground 427
Pin 4&6 no reading, number jumping around
Pin 2&8 to ground AC 939VAC (could this be right??)
Pin 4&6 to ground AC 359VAC
AC across pin 2&8 4.8VAC
AC across pin 4&6 724VAC

Filter caps
100uf -49.6
1st filter 426
2nd filter 425
3rd filter 363

I checked the top legs of the coupling caps for DC. All read zero except the .02uf phase inverter input cap which read 21V, and the two .1uf in the phase inverter read -39.5V. All the top legs of the coupling caps read zero for AC except the .02 phase inverter input cap which read 25VAC.

The only other thing to mention is even though I pulled the NFB wire to the speaker jack, the Presence pot read 1.75VDC just as it reads at the junction of the NFB resistor and the 1.5k in the NFB loop.

I apologize for the long post. I'm just at a loss at this point and would really appreciate some help from guys who know what they're doing. I've searched on this issue a lot and couldn't find much sounded just like it. I thought it might be coupling caps for a while, but I checked and it "seemed" they weren't the cause. Thank you.

[soundguruman]

The output bias is goofed up.
If you adjust the bias correctly it will work right.

[telesupro]

Okay, I'll check it when I get home. When the readings were taken it was biased at around 63% of max; 420V on the plates and 45ma. Is that bad?

[telesupro]

I have checked the amp biased really cold all the way up to 70%. I appreciate your response soundguruman, I just think maybe I'm too new at this to understand that there might be something wrong in the bias. Could you explain what numbers look strange? I know a lot of the voltages I listed are outside of spec and I'll be honest and say that I don't know why they're that way, if they're too far out, or what effect they would cause. Thanks!

[Enzo]

Telesupro, now that all your threads are available, please pick one, and let the other one end. we don't want to try to have a conersation in two threads.

Your voltages look fine, although I think you meant pin 3 not pin 2 in V2.


Do you know what a signal tracer is? You might make one and follow a signal through the amp, see where4 it fails.


Did you change ANYTHING between V2 and V3, where those tone controls are? Did you use a real 10 meg resistor in that local feedback?

I already suggested looking for an open ground path at a grid.

[telesupro]

Thanks for the guidance/suggestions Enzo. I tried to delete the other thread but missed the window; I'll let it die out.

I will check all the grid resistors and other signal to ground areas later tonight. You are right about my typo - I did mean pin 3 and not pin 2.

I didn't make any changes between V2 and V3, it's as per the original circuit. The 10 meg resistor is a NOS carbon comp. it measured within tolerance and I think I got it from Digikey.

I didn't know how to run, nor had I ever heard of a signal trace before you mentioned it (pitiful huh?). I knew guys used scopes sometimes in debugging, but didn't realize it could be done differently (and cheaper). I looked it up online and found a Billm article and video on how to make one and do the test. I already picked up the stuff from Radio Shack that I didn't have. I plan on putting it together tonight and running the trace. I'm kind of excited, but trying not to get my hopes up too high. I'd love to find it and fix it tonight though!!

Thank you again.

[Bruce / Mission Amps]

Also, triple check your wiring on V3... you might have lugs 7 and 8 reversed on either the board or socket.

[telesupro]

Thanks Bruce & Enzo. I checked pin7 and pin8 of V3 and where they attach to the board and they are like they're supposed to be, they appear to be anyway.

I checked all the components again and they are all correct and in the right places, again "they appear to be". I checked all my resistors and caps that lead to ground and the grounds "appear to be" good. I get zero resistance to ground. There is some possible grounding concern I'll talk about with the signal trace though.

I ran a signal trace last night also and kept running into what I thought was some weirdness around the last .1uf coupling cap (top and bottom leads), pin5 of V5, the 1M and 1.5k resistor in the phase inverter, and pin8 of V3. The weirdness that I got was that it sounded "open" like it wasn't grounded. Everything else in the amp had the signal I input at the instrument jack and sounded grounded. That area of the amp sounded the same as when I would put the probe on something that was ungrounded (like my arm or something). So the signal was present but the background sound was open.

Pin1 and pin5 of V5 was quite a bit hotter/louder than pin1 and pin5 of V4 also. It produced the signal at a much higher volume. To summarize: the .1uf coupling cap sounded open on both sides, pin8 of V3 sounded open, the 1M resistor sounded open on both sides, the 1.5k resistor sounded open on both sides, the 56K that they attach to sounded open on the bottom leg only, and the top leg of that 56k sounded grounded. I read zero resistance between the top leg of the 56K to ground.

I reflowed some solder joints in the area and it didn't fix it. I also lifted the top leg of the .1uf cap and checked it for DC. It bounced around a lot so I replaced it just in case and then checked everything again - still the same result. I played guitar through it again (quietly because it was late) and it was still the same terrible distortion sound. It maybe wasn't as gated, of course I was picking lightly, but I did notice some wah wah wah sound to it when I would strum a chord. It was ike I had a tremolo pedal on. Is that motorboating?

I don't know if any of the open sounding stuff is unusual or if that's just how it sounds in that area. This is the first time I've gone through an amp tracing the signal. Hopefully you guys can let me know what's standard and what's not. I wouldn't think an ungrounded sound in an amp is standard though. So that's what I have at this point and I'll mess with it some more tonight. Thanks to everyone again for the help, I need it!!

[telesupro]

Enzo, thank you for the suggestion for the signal trace! I did the trace with a tone as described above and it went all the way through. I did it last night with music for the signal and things were different. It was good up to pin2 on V2 but was bad at pin6 of V2. I wouldn't cross the 100k resistor from pin2 to pin6. I had checked that resistor before and it checked 112k. It checked 112k again, but it would not pass a clean signal. I replaced it and it solved one of the problems, the gated sound. The problem was a resistor that had an acceptable amount of resistance, passed a single frequency fine (my test tone 440Hz that I used), but didn't really pass a clean multi-frequency signal.

I don't know replacing the resistor fixed the signal all the way through the amp though since I managed to pop my little radio shack signal amplifier right after I found this problem.

Now I have ungated signal, but it's still distorted and I have low volume. It still won't get loud at all. I'm going to look at the filtering again. I think I've seen other threads around about low volume. I'll try to check those too.

[acorkos]

i have had brand new tube sockets with individual elements not making solid contact. i t wouldn't hurt to go through and make sure all socket elements are tight

[telesupro]

Thanks acorkos! I checked V2 socket and it seemed ok. Then I played through the amp again and it was back to the original problem. I thought I had gained some ground on the problem. I was wrong. I changed out the V2 socket just in case. No changes. I fixed my signal tracer and it still shows the signal being garbled coming from V2.

The signal sounds great at the wire that jumps pin1 and pin7; it's messed up thereafter. If anyone has any ideas I'd be glad to listen and investigate. I'm tempted to pull the board out and check the underboard wiring. Could something on the board cause this?

It appears to go into the first side of the V2 triode fine, come out fine, enter the second half of V2 fine, but then comes out of the second half of V2 messed up. Maybe I should break down and take it to a tech. I feel like I'm throwing things against the wall and hoping something sticks.

[Bruce / Mission Amps]

Did you block or are you sure DC is blocked from the cathode of the CF, the second half of V2 "B", which drives the tone stack and NFB?
Stock is a 100nF but anything from 22nF to 100nF works fine.

If not, the pots and resistors in those can bypass B+ over to the bass pot and on to the triode in front of the cathodyne driver.
Guess that would be lug 2 of V3 (A section).

[telesupro]

Thanks to all of you who helped out - the amp works! I still don't know exactly what was wrong, but after Bruce's post above I rechecked the 0.1uf cap. It didn't show any DC and it didn't before. I lifted the leg and the reading jumped around a little. I knew that the problem wasn't at the socket though and had to be in that area, so I replaced the 8uf electrolytic, the 100k, and the 0.1uf just to see...and it worked. I know it could have very well been a cold solder joint or something that looked ok, but really wasn't. So I'm not sure if I had a faulty component or some faulty workmanship, but now it works and I appreciate you guys taking the time to look at my problem and help out.