Vintage Modern 2266C - blown bias resistor

[TheKman76]

But, if I switch the tubes the problem moves to the other side. That sounds like mismatched tubes to me. Agree?

If the mismatch in bias looks identical and opposite, then yes.

That's a half-watt metal film resistor, right?

 

[gswearengin]

Yup, that’s what I got (well, it was listed as .6). I am tempted to try your idea of replacing both with something more robust, but this should be fine, right?

I ordered some Gold Lion KT66’s, as long as they really are in stock like it showed on the website. More expensive, but still less painful than buying yet a third set of Shuguangs! :$

Thanks,

Gary

 

[Pete Farrington]

When I set one side to 48mA, the other is around 36mA.

But, if I switch the tubes the problem moves to the other side. That sounds like mismatched tubes to me. Agree?

It’s probably the valves, but do you perceive it to be causing any actual issues?

Really, others will disagree but that discrepancy is bordering on negligible, it’s a guitar amp, not a missile guidance system.

Set the bias to 46mA with a new pair of matched KT66s.

That seems a bit high; what’s the typical HT voltage with these?
Bearing in mind that a design centre anode rating for KT66 would probably be around 22 - 23W, same as 5881.

 

[gswearengin]

It’s a guitar amp, not a missile guidance system.

Hey, speak for yourself there, pal!

No, I haven’t really noticed any ill effects. But since this amp is new to me and I just dumped a bunch of effort into combating a related issue, I would really like to hear it with well matched tubes.

But that’s also partially why I went with the Gold Lions…for a little variety instead of buying yet another set of TADs.

The 48mV bias value comes directly from the Marshall schematic. A 10% match is recommended, so one should be 48mV and the other should be 43 or greater. I’m at 36-ish, which is more like 25%.

I got a notification that the Gold Lions have shipped, so apparently they had them in stock. Hopefully it’s a good match! They’re pretty hard to find over here right now.

Thanks,

Gary

 

[Jon Snell]

It’s probably the valves, but do you perceive it to be causing any actual issues?

Really, others will disagree but that discrepancy is bordering on negligible, it’s a guitar amp, not a missile guidance system.


That seems a bit high; what’s the typical HT voltage with these?
Bearing in mind that a design centre anode rating for KT66 would probably be around 22 - 23W, same as 5881.

23mA per valve is around ball park ish figure.
The KT66 is a very hardy valve;

 

[gswearengin]

IT. IS. FINISHED. But not without a little more drama...

The Gold Lions arrived today. First of all, I realize that this is silly but OMG they are beautiful!

I managed to get the bias set to within 1-2mV of each other, which was such a relief after all of this drama.

And then...NOTHING. No sound. I realize I had plugged a speaker cable into the jack, but had forgotten I had disconnected the other end. I never really ran any real signal through it, but I was getting freaked out. Tried some other troubleshooting (different cab, different amp into same cab, etc.). Nothing. I don't have much time, so at this point I just have to leave it. So discouraged...

This evening, I finally get a chance to take a look at it, and: in a perfect storm of misfortune, one of the speaker leads had come disconnected in both of the cabinets that I tried!

I reconnect the speaker lead in the cabinet, put everything back together, fire it up and...THERE IT IS. Silly, but again, such a relief...

Thunderous low end from these tubes, even at low volume. I can see how this might turn people off, but I think I like it. I think I might also be getting a little more gain with the preamp volumes maxed (which I realize now makes no sense), which is a good thing if it's not just my imagination.

So, case closed, I think. I can finally embrace this amp as my own, hope for no more issues, and start playing around with other things like tube rolling. PHEW!

Thanks again to everyone for your help through what seems like a very long and unnecessarily complicated process.

Thanks,

Gary

 

[TheKman76]

The patient has made a full recovery then, excellent!

We'll bill you.