Recent Posts

91

Tube Amp Building - Tweaks - Repairs / Re: Fender Vibrato thumping

« Last post by AlNewman on October 17, 2025, 06:32:11 pm »

Alright, let's see about that.
So, -52V split at the bias pot = 52/32K = .0016A.
.0016 x 37K = -59.2 V
300 minus -59.2 = 359.2V differential.
359.2/3.2M = 0.00011A
0.00011 x 37K = 4.07V
Your math lines up.
Thanks for the lesson.

92

Tube Amp Building - Tweaks - Repairs / Re: 18W odd power problems

« Last post by BrianS on October 17, 2025, 05:21:47 pm »

When I said trace a signal, I should have said do you have a scope and signal generator.  Sounds like you do and are tracing the signal. 
I think in the past I have bypassed a non-functional stage just to assure myself that the rest of the amp is working and maybe dial down the trouble spot a little closer.  Not sure if that is necessary here or helpful at all really...
Good luck.

93

Tube Amp Building - Tweaks - Repairs / Re: Fender Silver Face 100 Watt Twin Hum

« Last post by dpm309 on October 17, 2025, 05:19:55 pm »

Wired a pair of 100ohm resistors to the pilot light and disconnected the humdinger pot. The has hum been significantly reduced. Will remove the 100ohm resistor from the PI circuit as Dave suggested and try to find a quieter grounding spot. Also hoping the the new filter caps will reduce the  hum even further. Looks like I am getting closer to fixing the hum issue.

94

Tube Amp Building - Tweaks - Repairs / Re: Weird distortion.

« Last post by Dave on October 17, 2025, 04:57:20 pm »

I run a variac into the primaries until I see 1 volt on the secondaries.


Dave

95

Tube Amp Building - Tweaks - Repairs / Re: Weird distortion.

« Last post by passaloutre on October 17, 2025, 04:54:37 pm »

The Hammond OT for "Plexi" type amps (https://www.hammfg.com/part/1750N) specifies a primary impedance of 3200R, so I'd expect a turns ratio to the 8R tap around 20:1. What are you using as an AC source for your measurents?

Sounds like you need a new OT. Or at least hook up a spare to prove it to yourself.

96

Tube Amp Building - Tweaks - Repairs / Re: Fender Silver Face 100 Watt Twin Hum

« Last post by mountainhick on October 17, 2025, 04:26:27 pm »

Yes, still getting hum when I removed V5. I have no idea what the extra trim pot for V2 was for. Disconnecting it from the circuit did not change the hum level. There is no center tap on the heaters and I do not see a pair of 100ohm resistors as shown in the schematic. I looks like they might of been connected to the ground lug located below the PT. Will wire a pair of 100ohm resistors to create an artificial center tap,

Pretty sure that pot is a humdinger and the center tap for your filaments. It may be the problem if there's crud built up in the contacts or the pot is failing. I had an identical setup on a stock 70's silverface bassman ten, took it out and replaced with two 100R resitors at the pilot light like 60's versions.

97

Tube Amp Building - Tweaks - Repairs / Re: 18W odd power problems

« Last post by Lectroid on October 17, 2025, 03:54:42 pm »

Sorry for the delay, chaps.  Been a busy couple of days.  Thanks for posting.

@tubeswell, B+ nodes:
2024:  A=325, B=314, C=292, D=275    These numbers I recorded last year, see the schematic.  Then last week, I measured these voltages:

2025:  A=374, B=364, C=324, D=303     

1. Why the sharp jump in voltages now? 
2. And why would the V1 plate voltages have dropped ~40V below what I recorded a year ago(?).  Both stages of the V1 tube have  plate voltages. lower than they were when I recorded the amp's voltages last year.

@BrianS,

Broken wire found.  Maybe a bad solder joint or other bad connection finally failed completely, or enough to cause the current problem.

Always a possibility with my soldering.   

I've been chop-sticking and continuity testing every trace I could but nothing found yet.  Ongoing. I've tried 4 or 5 12AU7 tubes in that slot and all display the same behavior.  IE, negative-function.

Can you trace an input signal?

No, that was actually my original problem, see above--no sound.  With a scope I see a test signal arrive at the amp's Instrument jack. I see the test signal arrive at the grid of V1b.  But from there, nothing comes out of that triode that I can detect.


Thanks!   

98

Tube Amp Building - Tweaks - Repairs / Re: Weird distortion.

« Last post by Dave on October 17, 2025, 02:58:06 pm »

Alright, I just tested a Super Reverb OT for turns ratio to get a ballpark comparison. I guess I must have forgotten what the numbers are supposed to look like.


Super Reverb runs two 6L6 at similar voltages to the EL34 amp I am working on.
Turns ratio = 44 to 1 --- 44X44X2 = 3872


7X7X8 = 392


Now I know that EL34 OT's run a little fewer turns than a 6L6 of similar design, but not 10 times the difference in total impedance.


I guess maybe it's safe to assume now that the OT is shot?


Dave

99

Tube Amp Building - Tweaks - Repairs / Re: Paraphase question

« Last post by goldstache on October 17, 2025, 02:51:23 pm »

Awesome thanks!
Honestly that’s what pushed me here. I cobbled in an ampeg r12-rb paraphase in, and found it to be noisy.
(Paraphase with input to 2nd triode derived from plate to plate divider, and slightly different cathode network.)


And that’s where the chassis is presently.


Noticed that it was noisier with the plate divided setup over the post coupling cap signal tap.


I get that they are different circuits, but AC wasn’t much different headed to power glass.


We see shunt caps across LTPI often. And even the ampeg r12-rb has a little shunt cap on the plate of first PI triode.


So I’m kinda thinking that the post PI coupling cap divider may be a more stable design.


The cross plate divider seemed to really up the idle noise in that stage.
So got me wondering why, if at all.


I realized that ground scheme plays a role here.
But rejection different between those two topologies?

100

Tube Amp Building - Tweaks - Repairs / Re: Weird distortion.

« Last post by Dave on October 17, 2025, 02:48:35 pm »

Alright, so I did a quick check of the OT without removing it.


It shows the following turns ratios...


4r  - 16 to 1
8r  - 7 to 1
16r - 10 to 1


I'm pretty sure that's not wired correctly, but I'll have to do the math to confirm. It shows 40 ohms across the primaries end to end. I didn't measure each one to the center tap.
Those numbers don't seem strange to me. Do they seem strange to you (other than the turns ratios being wired wrong)?


Dave