Ampeg B25 with EL34

[misterc57]

I recently picked up a B25. I think it is from the late 60s. It is rough cosmetically but intact and working with a small background hum. All graphics are gone from the face plate.

It was modified in 1996 to run EL34 (and similar 6L6, etc) output tubes.

My questions at this time are mostly sanity checking in nature. When I pulled the chassis out I noticed the following.

A 100K pot has been installed inside the amp to adjust (not sure what is being adjusted?) for the type of output tube used.

One of the 30uf 600v caps is leaking. I ordered a pair on eBay.

Pin 4 of each EL34 has a 1K 5w resistor, each resistor had a cold solder joint to the point where I was able to wiggle them off the pin. I can guess the weight of these resistors could have caused the solder joint to fail over time. One of the resistors is open. I will be ordering a pair of new resistors this week. I did not see these on the original schematic, I guess adding these was a good thing.

Once I get all my new parts installed I will take voltage readings on all the output pins and report back on those for your feedback.

Thank you

[g1]

The joint marked with an arrow is also suspect.


In general with these amps, it's a good idea to also resolder all the board mounted tube sockets.

[misterc57]

The joint marked with an arrow is also suspect.
[ATTACH=CONFIG]41879[/ATTACH]

In general with these amps, it's a good idea to also resolder all the board mounted tube sockets.

Good catch!

[Chuck H]

+1 to what g1 said.

The new resistors on the power tubes are a good thing.

The new pot is to adjust the bias voltage. It says "bias" on the chassis where it pokes out.

You should probably replace any electrolytic capacitor in the amp that looks to be stock rather than just those two big ones. I might even replace the newer one too since the amp was modded almost eleven years ago now and new electrolytic caps probably shouldn't be counted on for more than fifteen years. Might as well do the work while you're in there now.

[misterc57]

Cap?

I see this cap which has opened and sprung out its spiraled foil into the amp. It is .047 600 VDC. I am not able to find it on the schematic. Can someone point me to it?

Thank you



Schematic http://www.thetubestore.com/lib/thet...-Schematic.pdf

[Leo_Gnardo]

I see this cap which has opened and sprung out its spiraled foil into the amp. It is .047 600 VDC. I am not able to find it on the schematic. Can someone point me to it?

That's the "death cap". It died. Cut it out & throw it away & don't worry. Its function is to connect one side or the other of the AC line, through the cap, to the chassis, so you can get a nice shock. Part of your B25 fixup is putting on a grounded AC cable, right?

Time was, you could flip your so-called "ground" switch, whichever side got you the least hum was the right choice. Until you held your guitar & touched something else that was grounded for real, then you got a shock. Back to flipping the ground switch, no shock. With the chassis connected firmly to ground thru a proper AC cable, plug & grounded receptacle all that "ground switch" business is history. It's standard practice to ditch the death cap when installing grounded AC cable. Although for some unknown reason, manufacturers of modern grounded amps often insist on having one anyway. Must be "tradition, tradition!"

On the schematic, look at the lower right corner, you'll see "Ground Sw." and its associated 0.047 uF cap.

[misterc57]

Perfect!

This amp had previously been setup with a grounded 3 wire AC cable.

Thank you!

[misterc57]

Hum Gone But New Noise

My new 600v 30uf caps and 5w 1K resistors came in and are installed. All pots, tube pins and jacks have been sprayed clean. I found that the 100K bias trim pot is frozen.

Here are my EL34 voltage readings. I question if these are too high for EL34s. Pins 1 and 8 are soldered together.

V1 pin 2 and 7 = 6.35 VAC
V1 pin 3 = 513 VDC
V1 pin 4 = 509.7 VDC
V1 pin 5 = -50.7 VDC

V2 pin 2 and 7 = 6.35 VAC
V2 pin 3 = 516 VDC
V2 pin 4 = 508.3 VDC
V2 pin 5 = -50.9 VDC

The background hum is gone. I now have a new background noise. Sounds like radio reception static dial tuning or like if you inflated a small balloon and then kept squeezing the air out (on and off to make annoying noises).

This background noise is independent of the audio signal and happens with the volume controls at zero and no instrument plugged in.

Could this noise be from tubes or other caps?

Thank you

[Chuck H]

I've heard new tubes make noises like that when "breaking in". It usually goes away. But that doesn't rule out other possibilities for where the noise may be coming from. You have a busted bias supply. I would never plug new tubes into an amp with a busted bias supply. If the pot is "frozen" then it cannot be counted on. The tubes should be removed and the bias supply repaired before they are reinstalled.

It looks like you have a cold biased condition right now. Without a bias current figure we can't know for sure, but -51V is a lot of bias voltage. When the bias supply is repaired you can adjust it for proper current. That will load the voltage down some.

[Leo_Gnardo]

I now have a new background noise. Sounds like radio reception static dial tuning or like if you inflated a small balloon and then kept squeezing the air out (on and off to make annoying noises). This background noise is independent of the audio signal and happens with the volume controls at zero and no instrument plugged in. Could this noise be from tubes or other caps?

What happens if you remove the output drive tube? If your noises go away at that point suspect the drive stage.

Still using 7199 output drive tube? They're often the source of noises. New ones are rare, GOOD new ones even more rare, old good ones practically nonobtainium. Lately I've used a tube base scrambler (adapter) I get from Antique/CE that allows you to use other pentode/triode tubes for instance 6U8, 6U8A, 6GH8. Of course it makes sense to make sure that all the resistors and caps in the output drive circuit are good, not a bad idea to replace 'em. Also make sure the circuit board in that area is free from contamination, old flux & whatnot. Caps suspect especially if yours are the aqua blue or "tropical fish" types Ampeg liked to use. Also be alert high value (1 Meg +) resistors like to drift & sometimes open up with age, this amp is nearly half a century old.

[misterc57]

Yes it has the 7199 tube, a Sovtek. Noises were gone when removed.

I swapped it out with a nice old gold pin Realistic I had in my collection and the noise is gone! Now just have normal hiss sound.

[misterc57]

I will replace that frozen pot. Closest I had were 250K and 64K. Will have to order a 100k pot.

All the tubes in there are old and were likely replaced 20 years ago when the bias mod was done.

From what I have read those readings I had were close to what would be expected for the original 7027A tubes.

I have a bias adapter for my DVM, I will take a reading to see where it is currently at.

[Leo_Gnardo]

Yes it has the 7199 tube, a Sovtek. Noises were gone when removed. I swapped it out with a nice old gold pin Realistic I had in my collection and the noise is gone! Now just have normal hiss sound.

Yay! About 25 years ago there were some Sovtek/ElectroHarmonix 7199's. AFAIK none since then. Lucky if 1 out of 10 worked OK. Good to hear an old Shack part got your amp happy. Gold pins, how kool is that!

[misterc57]

Bias Current

Readings for the two EL34 were 27.5 mA and 14.8 mA. I swapped tubes around to be sure the numbers followed the tube and it did.

[misterc57]

Removed the 100k pot, got it working again and put her back in.

Installed two 6L6 tubes that matched at 30 mA current bias. Amp is sounding good. Will take new pin readings later.

Thank you!