Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Willabe on February 20, 2014, 11:56:33 am
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One of the bass players at church and I were talking and he asked if I could fix his Ampeg bass head. I said bring it to church and he did.
I got it home, opened it up and it looks like nobody has messed with it. Gonna need a full cap job. PCB looks very good with no sings of any burned up parts. The iron looks brand new absolutely not the smallest spot of rust on them to be found.
There's a 2nd 6A internal fuse in series with the external 3A fuse. The 6A internal fuse is blown so hopefully it saved the PT, I'll check for dcv later. The power tube socket furthest from the rectifier tube is burned, shorted heater to plate, pin 2/pin 3, so it needs to be replaced as it's plastic and probabley has a carbon track on it now. Surprisingly the 15A heater fuse is still good.
The GZ34's silver getter has turned white so it's vacuum is dead. The 2 power tubes are different as 1 is an RCA with a metal base and the other tube has no markings on it anymore but looks to have the same size plates, bottle, has a plastic base but it has no pins on the base omitted like a 7027A and they have different halos. The RCA has a top getter, other tube has top and side getter. I'm betting they both are 7027A's.
A few of the pots are pretty stiff, I'll hit them with some cleaner/lube and hopefully that will fix them. Otherwise I might have to rig new pots to work with the PCB that are hung on the pots connectors, shouldn't be too hard to do.
So as long as the iron set is still good I should be able to fix it.
The problem is that the schematic shows 500dcv on plates and 490dcv on the screens. I'll have to build 1 of those bucking transformer boxes like on Sluckeys web site just to get the dcv's back to the schematic's because with todays wall voltage they will be higher than that and there already at the limit.
I remember PRR saying that 7027A's were just 6L6GC's in (sometimes) a bigger bottle with a few extra pins. Triode electronics says the same thing, including that it was RCA marketing and that good/real 6L6GC's can handle the dcv's.
There's no screen grid R's and I'll add 1K/5watters. That should help.
Looked around for NOS 7027A's and NOS 6L6GC's in matched sets, they can be had for ~$100 (all the way up to ~$200 :w2:).
What's left of the Winged -C- 6L6GC's are around the same price.
Has anyone here used new production JJ 7027A's or 6L6GC's in an amp with these dcv's?
The iron set although they look like good quality they are not really that big. The back of the amp says 55w rms. They're definitely smaller than a BF SR set. Bobbins are wound 90 degrees to each other which is good. I don't think the PT will support 6550/KT66/KT88.
PT= H 3 3/4" x W 3 3/4" x L 4 1/2"
300707-1 89300690 682302
OT= H 3 3/4" x W 3 3/4" x L 3 1/8"
320823-1 8950025 682313
Here's the link to the schemo; http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_b25b.pdf (http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_b25b.pdf)
Brad :help:
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I have to say that I would be strongly tempted to use 6550s even I though I take your point about (I assume) much higher filament current. That would probably be wrong.
I would definitely put in a 5Y3, most especially during any troubleshooting...most especially if you have any lying around. That should whack your B+ by 20 or so volts. I always do so with *any* amp I am working on since I have plenty of them sitting around...you may not.
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Yes the heater current. The old Ampegs made by the original owner had oversized PT's that could handle the extra current draw. They even sold a "performance kit" that was 6550's and tube retainer clips.
I think this amp was made after they sold to new owners.
Good idea about popping in a 5Y3 at 1st, I also have a few 5V4's here.
Brad :icon_biggrin:
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The P/T's can handle kt-88's Wire for them, use a solid state rectifier and dont worry about the increase in voltage. 5y3 will not be able to handle the current of a pair of kt 88's. These amps are well built and are killer bass amps.
have fun.
Ernie
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The P/T's can handle kt-88's
Are you sure? Have you put them in a B25B and ran them for 6 months to a year?
I know the old Ampegs from New Jersey had over sized PT's but this 1's from after they sold. I posted the measurements and you can see from them it's pretty small for a PT that could supply 55w output (B+ plate current) and supply double the 7027A heater current.
Brad :dontknow:
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From Triode electronics web page: http://www.triodeel.com/7027.htm (http://www.triodeel.com/7027.htm)
Now, about them Ampegs...
Most likely you ran across this page after picking up an old Ampeg PortaFlex or V-series head or combo in a flea market or pawnshop. You liked the tone, in the case of the V amps, loved the cool tone controls, in the case of the Portaflex they look way cool, too.Then you looked under the hood, and ACCKKKK! The output tubes were 7027's! You called every guitar store & electronic shop in town: nobody has any.Someone said you might use 6550's, but they weren't sure..
Fear not, gentle reader, for the kind folks at Ampeg not onlyforesaw the problem and provided a simple solution, they made your Ampeg one of the most versatile amplifiers on Earth in regards to adapting to different output tubes! Years before Mesa & Crate even thought about it. You can pop in *any* good matched pair of 6L6-GC, (except skinny Chinese & Russian ones), 7581, 5881, 6L6-WGB, or KT66 without any modifications,
because the socket is actually wired for 6L6's.
6550, KT88, or KT90 may be used without modification, in most cases...(biggest problem is that the base clips usually won't fit KT88 or 6550) and by installing a simple jumper, both 6CA7 (fat bulb ) & EL34 (skinny bulb) tubes, too!
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here is another article about re tubing ampegs
http://www.ampegv4.com/7027.php (http://www.ampegv4.com/7027.php)
ernie
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JJ's 6L6's are perfect for this amp. I've used them in all the higher voltage Ampegs including those beasty 4B's.
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Thanks Ernie,
I have read the 1st thing you posted before. The Ampeg link was helpful.
I only found evidence that the V series can handle the extra heater draw from 6550/KT88 but they did say that the new JJ 6L6GC (7027A) would handle the 540dcv on the plates in the V series amps. I saw no mention of anyone changing to 6550/KT88 in a B25B.
Still you might be right. :dontknow:
Brad :icon_biggrin:
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Thanks Dan,
I figured you had probable messed wit at least a few of the high dcv old Ampegs before.
I'll tell my friend to buy a set.
Do you bias them a little on the cool side as their at a higher dcv? Say only 50% to 60% dissipation? He plays bass through it.
Brad :icon_biggrin:
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try this link
http://sunnforum.ampage.org/index.php?topic=2627.10;wap2 (http://sunnforum.ampage.org/index.php?topic=2627.10;wap2)
good luck,
ernie
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Thank you very much Ernie for the links. I really appreciate you taking the time to search them out and post them.
They brought up a new point in that last link that the 7027A's might have had a harder vacuum then 6L6GC's and because of that they lasted longer. :dontknow: Or was it that Ampeg ran the 7027A's cooler than most other amps with 6L6GC's? After all the owner of Ampeg didn't want any distortion from his amps so it would seem to make sense.
I remember Ken Fisher saying that RCA made the 7027A bulbs a little bigger and added the extra pins for a little more heat sink cooling to squeeze out 5 more watts to get the 7027A to 35w from the 6L6GC's 30w's. - But - than again not all 7027A's had the bigger bulbs and adding the pins very well could have been a marketing thing too.
That last link also said that most Ampegs had oversized PT's but did not give any info on which 1's didn't.
At least we know at this point that the JJ 6L6GC's will take the voltage and at not to bad a price.
Maybe I'll e-mail them over at flip tops and see what they say?
Thanks, Brad :icon_biggrin: