Ampeg B18-N Output Tubes Run Hot

[LBrewski]

I'm going over a very nice B18N that sounds great, is very quiet but heats up the power tubes, to the point that it deformed the plexi-glass lighted "Ampeg" sign attached to the tube shield. I have checked the voltages and they seem to be close to the values shown on the schematic, tried new output tubes.

I've read that it may be a good idea to replace the parts in the "capacitive-divider" bias circuit on general principal. The resistors measure good, the diode seems to as well. Should I shotgun-replace the capacitors? Any other ideas?

Other than being a bit dusty/dirty this amp is amazingly original:

Ampeg B18 pictures by ElBrewski - Photobucket

[txstrat]

I love those B-1XN amps. They sound just like I'd wish my bass amp to sound if Í were a bass player.
Anyway, an amp you wanna play should be in playable condition. Thatfore I'd do a capjob if it hasn't been already done.
Maybe some new output tubes gonna calm it a little. Depends on the tubes data.
Have a look at the tubes you can buy for Mesa amps for example. They have color codes for being hotter or cooler.
Hope this helps.

[g1]

Do you have around -55Vdc at pin 5 of the output tubes?
Check the 100K 2W resistor feeding the bias diode.

[LBrewski]

Do you have around -55Vdc at pin 5 of the output tubes?
Check the 100K 2W resistor feeding the bias diode.

Measures -49Vdc at both pin 5s. The 100k 2W measures 104k.

[LBrewski]

I love those B-1XN amps. They sound just like I'd wish my bass amp to sound if Í were a bass player.
Anyway, an amp you wanna play should be in playable condition. Thatfore I'd do a capjob if it hasn't been already done.
Maybe some new output tubes gonna calm it a little. Depends on the tubes data.
Have a look at the tubes you can buy for Mesa amps for example. They have color codes for being hotter or cooler.
Hope this helps.

If it were mine I'd make it a player as well, it still has a 2 prong AC cord and what appear to be original tubes (the output tubes have Ampeg logos). It's very "vintage", but also one of the quietest tube amps I've heard, so while I would change the caps normally, in this case I'm backing off because I'm getting this amp ready to sell to help a very nice lady who unfortunately lost her husband recently. He was a terrific bass player, missed by many. He actually had two of these, one went to a friend of his, another excellent player who gigs with it often.

I have tried another set of output tubes but they also heat up beyond what they should. I will look into cooler tubes but I need to make sure there something else going on.

[g1]

The drawing I have shows the output tube voltages at 505Vdc plates, 500V screens and -55V grids. You are about 10% low on the bias so to be proportional I would expect to see less on the plates and screens (approx. 450V?) if all is well.
I would suspect the 100uf bias filter cap. may be a bit leaky?
That extra -5V of bias should make it run cooler.

[LBrewski]

The drawing I have shows the output tube voltages at 505Vdc plates, 500V screens and -55V grids. You are about 10% low on the bias so to be proportional I would expect to see less on the plates and screens (approx. 450V?) if all is well.
I would suspect the 100uf bias filter cap. may be a bit leaky?
That extra -5V of bias should make it run cooler.

It was about 450V. I like the scenario of the leaking 100uf cap, makes since since it appears to be original, so I'll start with that one first. Thanks very much for your insight.

[corworld]

I was just tweaking the bias on my B-18 homebrew last night. 450 on the plates would definitely mean it is running hot. I ending up with 517 on the plates using a 42k resistor value on the divider to ground. The stock 47k schematic value pushed the plate value to 530. I need to put a 10k trimpot in there but it works well as is.

[LBrewski]

I replaced the 100uf cap in the bias circuit as suggested, the bias voltage now measures -54V instead of -49 and the amp seems to run cooler. Thanks to g-one for the suggestion.

Of course a complete cap job up front would have fixed this, but I'm happy to know exactly what the problem was (a 43 yr old electrolytic) without using the shotgun approach. Another vintage Ampeg lives on!

[txstrat]

Another vintage Ampeg lives on!

Great you sorted it out.

[g1]

Glad to hear it. I almost didn't mention it as you had stated the amp was quiet and I expected if that cap was bad you would have hum.
Now I know you can lose bias voltage via leaky bias filter before the hum gets too noticeable.

[LBrewski]

It's actually strange to me that this amp is so quiet; my recapped 66' Deluxe Reverb can't match it and I thought it was pretty quiet. I recently worked on a noisy Ampeg B25B, which has very similar circuitry but uses a pc board.