Ampeg Jet with 5881/6L6WGC instead of 7591

[Zaxxon]

I recently bought a '69 Jet that was very rough sounding, for a good price. The Jet is the one with black tolex and the big A, and a solid state rectifier. The seller thought it might need new tubes. I planned to take it to a tech, but thought maybe I'd try to get some new tubes for it first, to see if that would fix it. It takes two 12ax7s and two 7591s. I have a new 12ax7 that I tried in both the positions, with no difference made in sound, but I guess both of the ones in the amp could be bad. I read about these Tung Sol replacement 7591s being decent and wanted to order them:

https://www.tubedepot.com/products/tung-sol-7591a-new-production-power-vacuum-tube

But when I just pulled out the existing tubes, I found they are Sovtek 5881/6L6WGC. I know it's hard to tell without the amp present, but does anyone have any guess as to whether the different tubes might have been the problem, or if the amp has been modified to take these tubes?

 

[Lake Placid Blues]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

The old american 5881 has a plate dissipation of 23 watts and a max plate voltage rating of 400 volts. The 6L6WGB is the same tube but this the commercial part number while 5881 is the military part number.

The 6L6GC has a max plate dissipation of 30 watts and a max plate voltage of 500 volts. 7581 is the military part number. 7581A has max plate dissipation of 35 watts and a max plate and screens voltage exceeding 500 volts.

The old Sovtek 5881 is actually rated at 30 watts max plate dissipation and so is a GC. I mention this because if your amp calls for a 6L6GC or a 7581 then you should not use a 5881 or GB. But the Sovtek 5881 is the exception.

The 7591 must be a misprint because it only has a max plate dissipation of 19 watts or is actually a high plate voltage version of the 6L6GA from the 1930s.

http://www.radiomuseum.org/tubes/tube_7591.html

If your amp calls for a 6L6GC you need a 7581 military part number:

http://www.radiomuseum.org/tubes/tube_7581.html

6L6GA = 7591
6L6GB = 5881
6L6GC = 7581

The use of W in the commercial part number means it's extra tough or military grade.

 

[Lake Placid Blues]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

If the amp for sure calls for a 7591, then it may be not biased correctly because of the 19 watts plate dissipation vs the 30 watts plate dissipation.

 

[Lake Placid Blues]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

On further research the 7591 is not really = to the old 6L6GA other than the plate dissipation rating. It wouldn't hurt to get those new 7591s and try them. The will probably bias up better than the Sovteks.

 

[GilmourD]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

Doesn't the 7591 also have a different pinout to the standard 6L6/6V6/EL34/etc. setup?

 

[Lake Placid Blues]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

Yes it does. Good catch!

This means the amp has been modded to take 6L6s. It would need to be modded back to take 7591s.

What is the plate voltage? Tung Sol reissue 5881s might be a better fit if you don't want to mod it back, and if the voltages are not too high.

 

[Lake Placid Blues]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

If the 6L6s were installed without modding the amp, then that would certainly explain bad sound, if it even worked.

 

[MetalManiac]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

I also have an old Ampeg ( Mercury) that uses 7591 tubes. Old stock and eveen the new stock 7591's are pretty pricey- upside is they should last a long time, at least the old stock ones. I did find this- 7591 to 6GM5 adaptor converter. Not exactly sure what the point is- looks like they do not make any new stock 6GM5 ( which is a radio tube I guess), but they do look a bit less expensive than the NOS 7591's , and at least as common.;

http://www.ebay.com/itm/1pc-Gold-pl...530157?hash=item2c93d1536d:g:YNsAAOSwDNdV5vPA

 

[Zaxxon]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

Thanks everyone for the help. Thanks a lot Mojo's Minions for all of the research. So, is the case that both types of tubes can fit in the socket, but if it were modded for the 6L6s, what the pins are routed to is different, as well as other stuff changed inside the amp? And if it wasn't modded, the 6L6s would fit but the amp is configured/optimized for 7591s?

It seems like I have some interesting options, no matter what the configuration of the amp. I'm leaning towards getting it unmodded if its been modded. So if I order the 7591s to see if I can use the amp immediately, even if that doesn't work out, they could be put to use when the amp has been unmodded. On the other hand, maybe I should just take it in now, because it might end up being sooner overall that I can use the amp. Is it recommended to get the cord changed to 3 prong?

Thanks for the link, Metalmaniac. I was excited that the Tung Sol 7591s were available because I read that they were much better than the other new ones.

 

[Zaxxon]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

When the amp is used it buzzes and sounds scratchy, as well as kind of going in and out, almost like an echo or tremolo effect (tremolo off).

 

[GoldenVulture]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

When the amp is used it buzzes and sounds scratchy, as well as kind of going in and out, almost like an echo or tremolo effect (tremolo off).

Any idea if it's been recapped at all [ Power supply caps ] ? That's a very old amp and it's usuallly the first problem with old amps. Your description of the sound suits shot caps which then deliver a very unstable power feed introducing a lot of noise at the same time.

 

[Zaxxon]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

Thanks, GoldenVulture. That's good to know. I don't know if it's been recapped. Being it could be something besides the tubes, I'll definitely take it in. I hope he doesn't have to unmod it too, but I'll get it done if it needs it.

 

[Lake Placid Blues]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

Thanks everyone for the help. Thanks a lot Mojo's Minions for all of the research. So, is the case that both types of tubes can fit in the socket, but if it were modded for the 6L6s, what the pins are routed to is different, as well as other stuff changed inside the amp? And if it wasn't modded, the 6L6s would fit but the amp is configured/optimized for 7591s?

It seems like I have some interesting options, no matter what the configuration of the amp. I'm leaning towards getting it unmodded if its been modded. So if I order the 7591s to see if I can use the amp immediately, even if that doesn't work out, they could be put to use when the amp has been unmodded. On the other hand, maybe I should just take it in now, because it might end up being sooner overall that I can use the amp. Is it recommended to get the cord changed to 3 prong?

Go to the radio museum links I posted and print them. It has the pin outs for each type of tube. You then need to see which one your amp actually matches in its present state. You should probably locate a competent tech locally and have him go through the amp. He could do all the electrolytic caps, check the pin out, make sure it is safely grounded, and install tubes with them properly biased.

 

[Zaxxon]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

Sorry, should have been Lake Placid Blues, not mojo's minions, earlier. Would I have to open the amp up to do that? I'm not sure if I've got the knowhow for that. The diagram even may be beyond me. But I will definitely take the amp in and get all that stuff taken care of.

 

[Zaxxon]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

Just to follow up, I took the amp in; it needed 2 burnt power resistors replaced, and it had been modded for 6l6s. It was configured back to use 7591s. The amp sounds great.

Thanks for the help, everyone.

 

[jeremy]

Re: Ampeg Jet with 5881/6L6WGC instead of 7591

nice! those old jets are fun amps