EL84 mod questions

[whitelabrat]

The boards unscrew without a problem, but there are a lot of connections that need to be unsoldered first. So I'll have to document where everything is going. I figure I can put together a separate schematic for the boards and then the rest of the circuits that attach.

I'm tempted to pull out all the guts and put together new PCB's with a different circuit, components, and controls. The power and output transformers should be standard and easily reusable. Then I could solve a number of issues all at once.

I'm on a tight budget though, so kludges are the order for the day.

So here's my task list:

1. Use worn out 12AX7 in phono sockets. Can't beat that for a dummy load.
2. Replace ceramic cap stacks.
3. Hunt down other bad parts and solder joints and repair where needed.

Any thoughts on replacing other ceramic caps with a better material? Silver mica? Don't bother?

 

[gofar99]

Hi, My sense is that removal of the boards is probably going to be required regardless of all else. As you mentioned, there are two ways to go. Fix what you have or replace pretty much everything with new. Budget constraints may make the decision. Transformers, tubes and most of the power supply can probably be reused (filter caps probably not). The existing boards can be cleaned, parts checked and suspect ones replaced and /or upgraded. A related question is just how good was the original amp section. If it wasn't all that good in the first place, then the choice is easier. The second part of the process is what is your level of skill and knowledge. Rather than tie up a thread with back and forth details it might be easier to email direct (gofar99@hotmail.com) and make postings when something significant comes along. I'll be glad to assist either way.

Good listening
gofar99

PS I love your user ID, what part of Maryland are you in, I spent a few years in Silver Spring.

 

[whitelabrat]

Update. I initially resolved to gut and redo the circuit on this guy. This Calbest has an interesting design to it, and it may be worth restoring, just because of it's uniqueness. What I'll try to do instead is reverse engineer it and come up with a schematic. I'm making slow progress, so please be patient.

With regard to the power transformer, it has two outputs for the heater elements. I didn't bother measuring these. There is an output for the heaters in the 5U4GB putting out about 5.2Vac. Then there are the secondaries, pumping out about 690Vac to the rectifier. I measured about 450Vdc from the rectifier filament to the power filter cap. That's awful high for an 12AX7/EL84 amp so it must get dropped elsewhere in the circuit.

 

[gofar99]

Hi, Please see my separate email. Yes the voltage is rather high for an EL84. I suggest you try another meter to verify the value. I can't imagine a company putting in a transformer with that rating. Particularly for an older unit when premium EL84s didn't exist. Without special circuitry (fixed bias, CCS, etc), 450 would smoke the older ones. If I recall correctly the desired design rating is 300 on the plates and max rating isn't that much more for the earlier vintage tubes.

Anyone, if I err, please jump in.

Good listening
gofar99

 

[ThSpeakerDude88]

Did the unit say that it used EL84's or is that just what was in it? It could have originally come with 7199's... There are a few amps out there that originally came with 7199's and people unknowingly retubed them with 6BQ5's.

 

[whitelabrat]

There is a diagram on the chassis that indicates the use of EL84's and 12AX7's. Appears original. I thought about the possibility of 7199's.

The amp claims 30W per channel, which would indicate some sort of hot-rodding and may explain the high voltage.