Another new build with some startup problems

[Backlash]

Hi,
new express build done this weekend and I got amplification but alas not the expected kind. I’m hooking it to a 2x12 Marshall cab and yes it’s loud but not insanely loud at full throttle. I have no breakup at all, it just gets a little grittier at full volume.
Another strange behavior is that when the volume is turned all the way down, just when it hit stop it squeals like crazy – but when turned up just at tad it goes away? If I engage any of the capacitors on the bright switch – it squeals like crazy again?

Here is the voltage chart:
Mains 224 V AC
Secondary 577 V AC
B+1: 389 V
B+2: 380 V
B+3: 300 V
B+4: 285 V
B+5: 270 V
V1a-p1: 203 V
V1a-p3: 1.8 V
V1a-p4: 6.3 V (measured across p4 and p9)
V1b-p6: 175 V
V1b-p8: 1.4 V
V1b-p9: 6.3 V

V2a-p1: 253 V
V2a-p3: 2.6 V
V2a-p4: 6.3 V
V2b-p9: 6.3 V

V3a-p1: 191 V
V3a-p2: 25 V
V3a-p3: 38 V
V3a-p4: 6.3 V
V3b-p6: 193
V3b-p7: 23 V
V3b-p8: 38 V
V3b-p9: 6.3 V

V4-p2: 6.3 V
V4-p3: 387 V
V4-p4: 379 V
V4-p5: - 33V
V4-p7:6.3 V

V5-p2: 6.3 V
V5-p3: 381 V
V5-p4: 380 V
V5-p5: - 33V
V5-p7: 6.3 V

Does this seem to be correct? Any suggestions as to the amps behavior?

[jjman]

Have you tried switching the primary wires on the OP tube’s plates?

[fishy]

Any photos?

[Backlash]

fishy: I'll try to add some photos tonight.

jjman: Yes, the first thing I tried was switching the OT primaries and it squaled in every configuration. That also doesn't explain the lack of breakup... I have a feeling of what I'm hearing is only the outputtubes.. could that be it? Also, is the voltages on V3 correct?

[fishy]

V3 voltages look ok (check link)
V1 are either written down wrong or reversed.This happened on a recent build. Check the link for details on that too

https://tubeamparchive.com/viewtopic.php?t=6730

I have had a similar problem with a pot but it cleared up on its own when I cycled the pot several times. I replaced it anyway as a reliability concern. You can check the integrity of the pot and make sure it is low resistance when dialed down. Maybe it goes high resistance which is akin to kranking the volume.

[Backlash]

hmm, please pardon my ignorance but I'm not sure as to which pins on V1 you're referring to as wrong. I can't seem to find the information in the link you provided?

I have uploaded some pictures (yes, I know that the tube sockets are ¼ turned compared to normal layout ).

[gearhead]

V3 voltages look ok (check link)
V1 are either written down wrong or reversed.This happened on a recent build. Check the link for details on that too

I believe his V1 voltages are in the park. V1b (pins 6,7,8) is actually the input, with V1a after the tonestack.

Take a shot of your pots and post.

[fishy]

It appears I am wrong. Thanks for the recalibration....
Apologies for any confusion.

[Backlash]

Today I changed the offending volume pot, but the problem with squealing when turned all the down is still there???

The other problem with no break up - also still remains after switching tubes around and changing completely. It's very strange, I have very nice cleans and headroom all the way to 11 but I want tube breakup and lots of harmonics on the edge of feedback..Grrrr

Please help

[geetarpicker]

Bigger pics would help. A good pic of the pots would help too but still far enough away to be able to trace the leads back to the board. I suspect an issue with the control wiring. You should have alot of gain even on 1/2.

[Backlash]

I thought I owed you guys an update.

You were right on Glen.
After trying all sorts of more or less dubious stuff, I redid the tonestack form the ground and up. That did the trick!
Holy mother... I wanted gain, harmonics, sweet feedback and tube distortion... I got it all and a bit more than I can handle. I have to learn to control this beast. Best thing is there is a lot less hiss than in any of my other amps – and no hum at all. I’m very happy.
Thank you guys for your help and for making this forum the best on the web.


PeterM

[fishy]

Great news.....

Good job getting it resolved and your right about the guys on the forum.

[geetarpicker]

Yep, when I built my pair of clones one of them had an issue with the gain jumping around. There was one solder connection on one of the pots that appeared totally fine even close up under a 500w bench light, however I could tweak the chassis a little and get the gain to jump. I knew where the issue was but could not even see it. I also knew it was wired correctly as it would work fine most of the time. I decided to reflow it and all was fixed. One detail of using solid core wire is that a connection can look fine but actually be cracked.

Cool to hear it works now! Now time to tame it. Here is where you find out how good (or not) the pots are in your guitars, how well potted your pickups are, and how much your neighbors like you...