1979 Ampeg V2 blowing main fuse at full volume only....

[audiopete]

Yes, your bias is way hot. Stock was probably around 17W. If you put it back to stock values you could check where it was before (depending how much work that would be).
Your plate voltage shoud be somewhere around 585V. It is much lower because the tubes are biased hot. You should see it come up as you reduce the idle current.

Back when Sovtek came out with these (early '90's?), they confirmed that they were their 5881's with the extra 7027 pins added. I would assume they still are, which would explain why fliptops recommends them, as they are noted for ruggedness and being able to withstand high plate voltages.
FYI, the Ampeg V series (and probably all Ampeg calling for 7027's) do not require the extra pins that 7027's have, so any 6L6 that can handle the high plate voltages can be used instead of 7027's.

Excellent. They were running at 30mA stock originally and that is where they are now. I am about to do some testing. I will post an update but I expect everything will be fine now.

[audiopete]

OK biased up at 30mA and fired it up and played with pre volume at 10 and master volume at 10 and it sounded great and played for over 9 minutes and the fuse held.....then I saw one of the Sovteks start lighting up and failing, so I shut it down.

What is your recommendations for power tubes here? I have a set of Winged C 6L6GCs handy or should I go to a 6550 like EHX?

[Bruce / Mission Amps]

Amazingly, I have been having really good luck with the relatively inexpensive SOVTEK, 6L6WXT. Not sure what they are in the scheme of things from Russia but they sound pretty good at 485vdc and idling around 32ma.
Cooling yours down with more negative bias voltage will cause the plate voltage to rise. Well above 550vdc.
Keep that in mind.
I don't know if 6550s is the answer in that amp but I suspect it can handle those easily.
I've used JJ 6550s, SOVTEK's and the EH's versions... for the most part, they are fairly comparable.
I stopped using "C" Svetlana 6L6GCs a while ago due to way too many tubes with mechanical noise generated in the envelope. Same thing with the JJ 6L6GCs, actually they were worse....but in the past, both JJs and Svetlana's were always my first choices in modern 6L6 power tubes.
Somebody recently told me the modern Shuguang Chinese 6L6s were getting very good too.... have not been down that road yet myself.

[pdf64]

'I saw one of the Sovteks start lighting up and failing, so I shut it down'
If the screen grid current limiting resistors are still 470 ohms then such use may put the screen grids under significant stress / over dissipation.
The screen grid mesh probably overheated and sagged, shorting on to the plate or something.
Suggest that the resistors are increased to at least 1k 3w.
The Sovtek tube type were probably fine.
People have reported that the Sovtek 6L6 WXT+ is good for high voltage use, I think the EH 6L6 another name for that tube.
I've used them to good effect but not on a >500VB+ amp as yet.
Pete

[audiopete]

OK you are no doubt correct about the screen resistors. I will swap them to 1K 5W that I have and put new tubes in....still haven't decided what tubes but those 6L6WXT+ are looking promising...

Bruce - interesting...I found Winged Cs to be sturdier and quieter than 2 different pairs of NOS GE 6L6GC and a pair of Tung Sol 6L6GCs I a/B'd once in an old Supro Thunderbolt.

I'll see what the tube supplier recommends and go from there. Cheers everyone.

[überfuzz]

I don't appreciate the word "tinkered" and what it infers, my friend. I do this (restore and repair music electronics, not just tube amps) for a living, and only rarely do I need to come here for advice or help - which is greatly appreciated. I assume you now know the answer to your second question.

If you meant "tinkered" in a benign manner then please accept my apologies.

Didnt mean to qestion your abilities. I like the word tinker. By the way, it seems it was one of the obious things after all, didnt it. Cheers!

[audiopete]

Didnt mean to qestion your abilities. I like the word tinker. By the way, it seems it was one of the obious things after all, didnt it. Cheers!

It's hard to know intent sometimes communicating this way in print....thank you for your clarification and your advice offered. Cheers and please tinker away!

[R.G.]

Check the values of the grid leak resistors, too.

As tubes get hot, the combined internal effects of imperfect vacuum and surface conditions on the cathode and grid can lead to increased grid leakage. If the resistors are too big to eat the increased leakage, this increases the effective bias and the tube turn on more. It's a form of thermal runaway. This is one of the reasons for a specification of the maximum value of grid leak resistors in tube datasheets.

A tube already hot can be running on the edge of runaway. Turning down the bias reduces the average temperature. Reducing the grid leak resistors makes them more resistant to runaway.

[R.G.]

Re: Tinker

I always think of "tinker" as either a neutral word, implying doing small changes to something as opposed to massively changing it; or as a positive reference to the people who were the source of the word.

I haven't done a true search on the origins, but my understanding is that tinkers were people who repaired household metal objects, especially pots and pans. They did this with almost no tools available. The term "tinker's dam" as I remember was a reference to the practice of making a dam (i.e. thing to obstruct fluid flow) out of mud to keep the work localized. They did useful, if small, work on things with minimal tools.

Somewhat along those lines, I think of the word "mod" as perversely pejorative. "Mod" as it's become used, is viewed as a change that any uneducated, unskilled user of an amp or pedal can do that will make his equipment sound like lasers, flying saucers, or their current guitar god. Probably every tech has opened up a "modded" amp and wondered how it kept from exploding or going up in flames. We identified a quasi-medical condition on one of the pedal forums, the key symptom being a user's first or second posting to the forum being something like "Hey doods! Anybody got some hot mods for a Belchfire pedal?"

We called it BUMS - Blind Urge to Mod Syndrome.

[audiopete]

So R35 and R36 (100K) should be lowered if there are still issues? What value would you recommend?

I am confident that the cooler bias and fresh tubes will sort this amp out.

The more I thought about it, the more I realized that there are different expressions from different regions and we are truly on a world-wide forum....I'd imagine "tinker" is more popular a word in the UK.

I think of "mod" as "modify" meaning to change from stock. I hear you on the senseless mod syndrome (SMS)...some might call it "circuitbending" as in changing things and not knowing a damn thing about what they are doing.....it either produces a funny result and you like it, or it blows up on power up.

[gtrplayr1976]

30 watt max per tube, according to Sovtek

The only reason I mentioned that is because I have JJs rated at a max 30 watt ,but they say bias as a 25 watt. Sounds like you have it figured out already though

[audiopete]

Just to finish this off, the amp is back with its owner and running great and sounding even better. It's a loud, mean sound - thick cleans and raunchy crunch, and now I have to get one.

Thanks again everyone for your time and skill.

[g1]

Could you tell us which type of power tubes you ended up using?
Thanks

[audiopete]

Could you tell us which type of power tubes you ended up using?
Thanks

Sorry for the late answer - I went with Winged C 6LGCs and they are still rocking today. Can't say enough good things about Winged C. Let's hope the rumours are wrong and they keep making guitar tubes.

[Mick Bailey]

I always think of "tinker" as either a neutral word, implying doing small changes to something as opposed to massively changing it; or as a positive reference to the people who were the source of the word.

I'm with the bunch that sees red with the word. To me it's come to mean someone unskilled who randomly plays about with something in the hope of achieving a result, without actually knowing anything about what they're doing. My Father's generation certainly uses it to belittle people. Funny, but 'Techies' gets the same red mist. I had a team of 27 highly skilled programmers and systems analysts - many with Masters degrees in technical or business fields. I often noted the derisory stress applied to the word in order to demean technical staff working in a business environment.