twin reverb, power tube current, etc.

[CraigB]

Hi, been awhile since I've posted here.  Sort of sidetracked on building a guitar, so haven't been doing any amp-building or repairs.

Now for a question or two for you Fender amp experts.  I just got a SF Twin Reverb.  Recapped power supply.  ditched the "bias balance" and other CBS-type "accessories".  Added some of those 1R resistors Doug sells.  Biased each tube to around 35mA, but one of them sits at about 40mA.  They're Sovtek 5881s.  Is this OK for now until I can afford a matched quartet?  Any recommendations on 6L6s?

So I don't have any snaps, crackles or pops after playing it at moderate to loud volume for a few hours over the weekend.  Trem works.  Reverb works, but seems a little bit "weak", if that makes sense, i.e., doesn't do the classic surf-verb.  I've tried some different 12AT7s, doesn't seem to matter much.

Other than what I mentioned, are there any caps, resistors, other components that you absolutely replace when servicing for customers?

Thanks!  Craig

Ah, I'm reading the servicing Fender amps section.  Sorry, moderator, feel free to delete this thread!

[Danskman]

Hi,
I would replace all the 25muF/25V preamp cathode bias caps (aren't they plastic molded white?), as they tend to be defective after all these years; I did it in a SF Bman 100 some time ago and it was worth the pain and the $$ .
Regarding the bias, did you try to set it a little bit hotter (around 40-45mA)? Maybe the difference between the 4 tubes could be lower when they are biased hotter. If it doesn't change, you'll have to get another closer matched set. But  a difference of 5mA on one tube is not as bad as it seems, IMHO.
HTH and BR,
Danskman

[PRR]

> around 35mA, but one of them sits at about 40mA.

I would not worry. EVER.

If it were a pair, and I "had" to get a low-low THD measurement, I'd use another pair. But gitar is not about low-low THD. And in a quad, the 14% difference in one bottle averages with its mate to be a 7% difference. And 50% differences don't really hurt the distortion. Certainly not if you are going to clip it. (And if you are not taking a Twin to clipping, you should carry a smaller amp.)

[FYL]

And if you are not taking a Twin to clipping, you should carry a smaller amp.

Dick Dale, is that you ?
 

[CraigB]

 

No kidding, that thing is a beast.  I don't want to carry it anywhere! 

[CraigB]

Just in case this helps someone else.  I noticed that the reverb was rather weak, it's there, but when you turn up the control, it introduces noise, and still not the the classic surf you exepct from one of these.  Pulled the RCAs and measured the coils - they were both about 150 ohms.  Uh-oh, wrong tank!  Swapped in the right 8 ohm tank from my PR and now it's working perfect, and quiet.

Also replaced all cathode caps and the 470R and 1.5Ks on the power tube sockets.

It seems like the trem depth control needs to get turned up a little further than I'd like.  I know somewhere I've read about a potential fix for that (assuming that the opto-isolator is still good) but I've googled/searched and can't' find the article I'm looking for.  Anyone know of some things I can try to remedy that?

Lastly, it actually sounds good with the Sovtek 5881s.  But I'm going to try the EH 6L6s in it.  Anyone have any experience with those?  I've never tried them in anything.


Thanks - Craig

[mresistor]

I use EH 6L6s in my 78 PR and they sound great to me.