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

[audiopete]

Hello, I have an Ampeg V2 in for service and it is blowing the main fuse after about 5 mins of full level playing. On the bench at about 60% level it operates for hours.

I just did a complete cap job, new tube set (mostly NOS, but new Sovtek 7027s, new Tung Sol 12AX7s), repaired open standby pilot light dropping resistor and open reverb recovery plate resistor, bias modded to adjustable and bias set to 42mAk/550Vp, voltages look good and the amp plays and sounds great until you open it right up to 10 on pre volumes and master volume. After a few minutes of continuous playing at full level it dies with a blown fuse. 7027s tested fine on my B&K747 but I haven't tested them yet after this second incident.

My next step is to swap out rectifier and flyback diodes and look for arc'd sockets, but there are no pops or crackles so I don't suspect an arc.

Perhaps the Sovteks just don't like this amp and it's high B+? Fliptops recommends them for Ampegs, so I am not convinced the tubes are to blame. Diodes test ok at tester voltages but they are original and operating at serious voltages, so perhaps they go short at the intense load of full volume?

Any thoughts? Schem attached although the amp I have is a later version with MV and large main filter caps. Very similar other than that.


Thank you very much.

audioPete

[Bruce / Mission Amps]

Saying "full volume" means almost nothing.
Nobody here knows what that is or what you think full volume is.

Personally, I would NEVER run any Sovtek 7027s at "full volume" when idling at 23 watts each in this amp.... 42ma@550vdc.
That's crazy talk... besides, I'm not sure those Sovtek 7027s even are 7027s!!

Try resetting those power tubes down to around 30ma each and see if it still blows.

[pdf64]

By full volume, do you mean it's maximum unclipped output, or it's maximum clipped output, ie max sin or max square?
What line current does it draw under those conditions?
Pete

[Jazz P Bass]

#1 lower the bias.
#2 are you using a 4 amp slo blo fuse?

[überfuzz]

Did it blow fuses before you tinkered with it and are you using the right kind of fuse?

[gtrplayr1976]

Wow...are those 25 watt tubes ? If so that's almost 95% bias.

[audiopete]

Saying "full volume" means almost nothing.
Nobody here knows what that is or what you think full volume is.

Personally, I would NEVER run any Sovtek 7027s at "full volume" when idling at 23 watts each in this amp.... 42ma@550vdc.
That's crazy talk... besides, I'm not sure those Sovtek 7027s even are 7027s!!

Try resetting those power tubes down to around 30ma each and see if it still blows.

Thanks Bruce, good ol Weber bias calculator....didn't do the math...my bad.

Full volume means volume pot and master volume fully open to 10...thought that was clear..

Anyway I'll try that...makes sense

[audiopete]

#1 lower the bias.
#2 are you using a 4 amp slo blo fuse?

fast blo, but never blew fuses before apparently

Doesn't specify slo blow on the rear panel.

[audiopete]

Wow...are those 25 watt tubes ? If so that's almost 95% bias.

30 watt max per tube, according to Sovtek

[audiopete]

Did it blow fuses before you tinkered with it and are you using the right kind of fuse?

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.

[pdf64]

'Full volume means volume pot and master volume fully open to 10...thought that was clear.'
So the amp is being heavily overdriven?

[audiopete]

'Full volume means volume pot and master volume fully open to 10...thought that was clear.'
So the amp is being heavily overdriven?

Yes, like any young heavy band would run it.....

[pdf64]

It would be a good idea to check the line current then.
The designer may not have envisaged it being used like that.
Also what's your line voltage?
If it's above what it was designed for, it will draw more current.
Generally, T rated / slo blo / time delay fuses are used for inductive loads, such as transformers, motors etc; in order to accommodate the current surge at switch on, not that it seems directly applicable to it blowing under heavy load.
Pete

[audiopete]

It would be a good idea to check the line current then.
The designer may not have envisaged it being used like that.
Also what's your line voltage?
If it's above what it was designed for, it will draw more current.
Generally, T rated / slo blo / time delay fuses are used for inductive loads, such as transformers, motors etc; in order to accommodate the current surge at switch on, not that it seems directly applicable to it blowing under heavy load.
Pete

Thanks for your help. Line voltage on bench iso transformer/variac is 120VAC. I will bias colder and I think that will correct the issue. The fast blo 4A fuse that came with it looked pretty old and original, so I don't think it's been pushed like that before to blow a fuse.

[g1]

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.

besides, I'm not sure those Sovtek 7027s even are 7027s!!

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.