135w Ultralinear "1/2 Blackface" Mod

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As you’re not pushing much power out of the amp, the power valves shouldn’t wear much, and higher value screen grid resistors wouldn’t therefore help to reduce the wear any further.
Only 6-8 months life out of a set seems disappointing, what type from which manufacturer from which vendor at what idle anode / cathode current?
Have you got the load impedance right for that funny output switching jack thing.

[22uf]

As you’re not pushing much power out of the amp, the power valves shouldn’t wear much, and higher value screen grid resistors wouldn’t therefore help to reduce the wear any further.

Ahh ok. Thanks for the heads up.

Only 6-8 months life out of a set seems disappointing, what type from which manufacturer from which vendor at what idle anode / cathode current?
Have you got the load impedance right for that funny output switching jack thing.

Well I contacted the previous owner and he said the power tubes were actually 5 years old! So I was getting excited over nothing.. 
I can confirm that I have been very careful with the jack. I cleaned it carefully and I check it every time the amp comes out the cab.
The transformer on these things seems pretty tough though. A somewhat rubbish roadie friend failed to remember to hook it up to an external cab and ran it without any load for a good 2 hrs last weekend! Seemingly unaffected. Thank goodness. He has since been euthanised. 

I have another question with which to bore the hole off you, if you'll indulge me..

I have been experimenting with bright caps and I have found that I really like the sound of both the stock 105pf (as measured) cap and no cap. No cap the amp sounds honestly much warmer and neutral EQ. It really honks for lead and especially with the channel volume on 10.I'd really like to switch between the two and since I have a push/pull master volume (with a "boost") that I never use, I was wondering if I could put the bright cap on the switch instead of the boost?

I took note of the fact that my cheap crocodile clips had 1.8kohm resistance so I have a 1.8k carbon comp resistor which I will wire in parallel with the 105pf cap.

Here's a shot of the back of my volume pot:

https://photos.app.goo.gl/aJSgoWKreSsqTf3w6

Am I right in thinking the boost is hooked up under the yellow circle? Can I get away with just de-soldering those wires and tape them off and replace them with the cap/resistor?

PS: If anyone cares I have pulled the trem tube (it thins out the tone even further) and put an NOS RFT in v2 and a JJ ECC83S in V1 (which adds a nice warm/midrange sound to the gain). I have kept the stock speakers (Eminence Legend 1258s) as they really work well with this amp. I use an OCD 1.7 to just gently push the preamp in to compression. It works wonderfully and is, together with the bright-cap ON clean tone, absolutely the best of both worlds imho. I'm really quite certain all that I really need to play any venue is a twin reverb and a tubescreamer!

Edit-Edit: I just learned how cathode bypass caps work hehe. I think I'm going about it the wrong way above ^. Perhaps might be better to use the push pull to cut the tremolo intensity pot out of the loop, so I can still have a working tremolo if required without the tone suck.

Then either re-voice V1 for higher gain or put a rotary in V2 low input (which I never use) that has some different cathode bypass values for a bit/lot more preamp gain.

This video and the linked app seem like a good place to start!

Is there any reason I can't tweak the cathode bypass cap values in this amp? It won't tear a hole in space time or anything?

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Great, buying the same valve brand from the same vendor may be a good idea then!
How were you using the amp, without anything plugged into its output jacks?
Whatever, unless it was turned up enough to overdrive the power amp, as there’s a negative feedback loop, that should help to prevent problems.

Regarding the bright switch, I can’t envisage how croc clips could add 1.8k resistance 
May measure 1.8ohms, but that would probably be in error, attributable to the probe leads.

The pull switch is DPST, ie 2 pole, and as I recall, the terminals you’ve circled may be different poles. Rather the switching contacts may be aligned vertically.
It should be ok to insulate off unused wires, but be careful not to tuck them out of the way and unwittingly compromise the lead dress. 

Are you sure about the trem valve V5 affecting tone? As it’s completely out of the regular signal path, I wouldn’t suspect it of doing so.
Whatever, it’s fine to leave it out.

[22uf]

Great, buying the same valve brand from the same vendor may be a good idea then!
How were you using the amp, without anything plugged into its output jacks?
Whatever, unless it was turned up enough to overdrive the power amp, as there’s a negative feedback loop, that should help to prevent problems.

I wasn't, he was supposed to have patched it in to an external cab but only managed to disconnect the internal speakers before getting distracted.
MV was on the usual 4.5 the whole time. I guess I got lucky.

Regarding the bright switch, I can’t envisage how croc clips could add 1.8k resistance 
May measure 1.8ohms, but that would probably be in error, attributable to the probe leads.

Ahh OK. I should have used my multi meter but I used a capacitance tester instead which reads wonky if things aren't a capacitor or a resistor or a diode.
I'll check that hehe. Thanks for the heads up.

The pull switch is DPST, ie 2 pole, and as I recall, the terminals you’ve circled may be different poles. Rather the switching contacts may be aligned vertically.
It should be ok to insulate off unused wires, but be careful not to tuck them out of the way and unwittingly compromise the lead dress. 

Thanks. I'll stop being lazy and look at the schematic! 

Are you sure about the trem valve V5 affecting tone? As it’s completely out of the regular signal path, I wouldn’t suspect it of doing so.
Whatever, it’s fine to leave it out.

Absolutely positive. Having V5 in changes the tone of the amp. Its subtle but it definitely loses a bit of pre-amp gain.

So all I want from this amp is for the preamp to have more gain from around 7 on. I don't need all this headroom and on 10 (mv 4-5) it really starts to sound like its going to "go", but it doesn't quite get over the line. I'm not greedy, I just want another 2-3db of gain off the preamp OR some power amp breakup. I play mainly blues jump type stuff and the amp is so so close my perfect lead tone:

Its 90% of the way there without a pedal and sounds way better straight in through a curly cable.

I'm totally happy with the bass/treble response and the overall clean tone of the amp is fantastic. I just need a hair more gain. Pulling v1 gives more gain but its thin and nasally. Goosing the reverb a bit does too but I don't always play with reverb on "wet".

After reading lots of internet posts I'm left thinking the only way to get more distortion (if there's no way to get it from the preamp alone) without fundamentally changing the clean/character of the existing tone is to reduce the blocking in the power section by somehow getting the global NFB hooked up to a pot? This would let me get some power tube breakup (I say SOME, I have no intention of going nuts) which might give me what I am looking for?

[2deaf]

The pull switch is DPST, ie 2 pole, and as I recall, the terminals you’ve circled may be different poles. Rather the switching contacts may be aligned vertically.

The switches are aligned vertically if you are holding the pot in your hand with the lugs up.  However, the pot has been rotated 90 degrees in the amp so that the switches are aligned horizontally i.e. the top two terminals that are circled comprise one of the switches.

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The pull switch is DPST, ie 2 pole, and as I recall, the terminals you’ve circled may be different poles. Rather the switching contacts may be aligned vertically.

The switches are aligned vertically if you are holding the pot in your hand with the lugs up.  However, the pot has been rotated 90 degrees in the amp so that the switches are aligned horizontally i.e. the top two terminals that are circled comprise one of the switches.

Well spotted! 

...he was supposed to have patched it in to an external cab but only managed to disconnect the internal speakers before getting distracted.
MV was on the usual 4.5 the whole time. I guess I got lucky...

How did your hear your guitar then 

...
So all I want from this amp is for the preamp to have more gain from around 7 on. I don't need all this headroom and on 10 (mv 4-5) it really starts to sound like its going to "go", but it doesn't quite get over the line. I'm not greedy, I just want another 2-3db of gain off the preamp ...

If you’re not using the reverb on full, then a few dB more preamp gain can be achieved by reducing the value of the 3M3 dry mixer resistor, eg 2M2, 1M5, 1M.

...After reading lots of internet posts I'm left thinking the only way to get more distortion (if there's no way to get it from the preamp alone) without fundamentally changing the clean/character of the existing tone is to reduce the blocking in the power section by somehow getting the global NFB hooked up to a pot? This would let me get some power tube breakup...

Increasing the power amp gain is pointless, as you’ll just reduce the master volume setting. Power amp overdrive won’t happen until, err, the power amp is pushing out more than 135W (or whatever it happens to actually be managing in the absence of a fresh set of Sylvania STR387).
Which, as you indicate, is probably a lot louder than you’re playing.

[22uf]

I wasn't playing. The amp was set up on stage prior to the gig. He disconnected the internal speakers and was supposed to plug it in to an external 4x12 but didn't manage that part. 

Increasing the power amp gain is pointless, as you’ll just reduce the master volume setting. Power amp overdrive won’t happen until, err, the power amp is pushing out more than 135W (or whatever it happens to actually be managing in the absence of a fresh set of Sylvania STR387).
Which, as you indicate, is probably a lot louder than you’re playing.

Ho hum. Thanks again for letting me know. I don't really understand any of this you know. I just know what I want and have just about enough of a clue not to electrocute myself.

If you’re not using the reverb on full, then a few dB more preamp gain can be achieved by reducing the value of the 3M3 dry mixer resistor, eg 2M2, 1M5, 1M.

I'll give that a try and report back. If that doesn't work I'll try some different values in place of the 820ohm NFB resistor. Or, put it on a pot.

And if neither works.. I'll go buy a pro reverb. God dammit. 

Edit: Looks like deleting the "distortion " circuit can actually add .5-1 db or so as well. Poof!

[Willabe]

If you want less power output/volume, pull 2 of the 4 power tubes. Just run the amp with a single pair of power tubes.

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I wasn't playing. The amp was set up on stage prior to the gig. He disconnected the internal speakers and was supposed to plug it in to an external 4x12 but didn't manage that part. 

As there was no signal there shouldn't have been a problem; the shorting contact on the output jack, or failing that, the NFB loop, should have prevented anything too bad happening.

...Looks like deleting the "distortion " circuit can actually add .5-1 db or so as well. Poof!

I don't see how?
Oh, I think that the the 2 poles on the master vol pull switch work in opposition, ie as the switch is actuated out and in, when one pole opens, the other closes, and visa versa.

[22uf]

I wasn't playing. The amp was set up on stage prior to the gig. He disconnected the internal speakers and was supposed to plug it in to an external 4x12 but didn't manage that part. 

As there was no signal there shouldn't have been a problem; the shorting contact on the output jack, or failing that, the NFB loop, should have prevented anything too bad happening.

Thank goodness!

...Looks like deleting the "distortion " circuit can actually add .5-1 db or so as well. Poof!

I don't see how?
Oh, I think that the the 2 poles on the master vol pull switch work in opposition, ie as the switch is actuated out and in, when one pole opens, the other closes, and visa versa.

Internet "wisdom". I can't find the link grr.

Anyway I tried the reverb circuit changes (no cap/lower value resistor etc). More gain for sure. Sounded like ass. Might as well be a solid state amp. Real wasp in a jam jar stuff.

I've since put it all back to stock.

I concede. It is what it is. A loud clean amplifier. Fin.

I can get it where I want it to be straight in with some old super distortions in a 335. So there's that.

Honestly I think I just like tube rectified amps. I like the combination of touch/feel/tone when you float like a feather and scream bloody murder..

I'll keep the twin (ordered a JAN WC for the PI) and respect its beliefs. ie. No sex ever. 

[22uf]

JAN 12AT7 (GE) arrived. Not a good fit for the PI. Clearly very imbalanced triodes. I have ordered up a tested balanced JJ12AT7 for the PI.

Put it in V3 instead. Man that tube gets HOT. Is that normal? Its too hot to handle after 5-10m.

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Seems that your vintage valve has a triode section that’s basically dead. Yep, even the best don’t last forever, if it was advertised as working then may have been missold.
The valve in V3 is a class A power amp, so it will run hot. But shared cathode bias arrangements rely on all valves being at least vaguely functional; if one isn’t the others will be underbiased (ie insufficient bias voltage) and consequently run hot.