Dropping the B+ voltages....

[Paultergeist]

Greetings,

I am working on an old and rather obscure amp from the early 1960s -- it is called a Tiesco (brand) "Checkmate 20." Schematic if anyone is interested:

http://www.larryjohnmcnally.com/Teisco/ckmt20_sch.pdf

If memory serves me correctly, it had about 380 volts on the power tube (7189A) plates during the brief moment in time when I got the thing fired up. It needs -- I suspect/believe -- a cap job, and I am in the process of performing a complete replacement, but other things are coming up along the way.

The power tubes (original?) are "7189A" -- which are pretty dang close to an EL84 save for a minor pin-out adjustment. 7189As are nearly extinct. The issue is the plate voltages -- 380V is too high for a regular EL84; maybe I could use the EL84Ms, but they are much less common, and the owner would like to be able to use common EL84s. I need to lower the B+ voltage to the power tubes......easiest way to go?

Add another big ol' power resistor off of the rectifier circuit?

How about the 5 Ohm resistor to ground off of the "voltage doubler" rectifier -- between the two diodes? Can I increase the value of that resistor to lower B+?

Any thoughts are most appreciated.

[eniam rognab]

hi, im running into the same problem right now using old hammond iron for a rocket build. i am using a large zener diode on the HT centertap to drop the voltages. google "zener diode on center tap" for some good reading and info on the parallel cap

disadvantages are cost (pricey buggers), some added switching noise that the cap is used to cut down on, and probably some other stuff that someone else can enlighten you on. i know some guys around here dont like this method but its something for you to ponder

have fun!
neal

[Paultergeist]

Thanks, Neal.

I have heard of that which you describe, but I do not think this particular amp has a HT Center-Tap. (I could be wrong, but that is how I have come to interpret the circuit). I do appreciate your reply, however.

I'll probably just use "brute force" and power-resistor the B+ down to where it needs to go.......it is not an elegant solution, but it is what I at least know how to do.

Best of luck with your Rocket build.

[eniam rognab]

i prolly should have looked at the scheme you linked huh? its a voltage doubler and i am fuzzy on such things....

haha yup nothing there to zenerize, maybe try a VVR?

resistors are probably a good solution, like a big voltage divider like i did in my first amp. i just want to avoid this if i can cuz it generates a lot of heat and i think its inelegant... but sometimes the best is to KISS keep it simple smart

[pdf64]

A zener would fine if place in series with the dc output from the rectifier.
But yes heat will be generated.
How about converting it to a 6V6?
Pete

[rp]

IIRC (real) 7189A were a higher dissipation version of el84s and should be OK at 380V, assuming they are still strong, and I bet the amp was designed that way and not out of wack. It's a pricey tube NOS. Problem is in the future or if someone swaps for fun or has a higher wall voltage than you. Supposedly there are new (JJs? The fat ones?) that are like 7189s and can take the higher voltage, others familiar can pitch in. If it's for you I would just check the tubes in the dark, bias a bit lower, and use the orig tubes or the modern high power version.

Just re-read that it's a repair. If it's not red plating just tell the owner about the need for the right power tubes and leave it. Put a sticker inside the cab warning to use only the recommend EL84s.

[Paultergeist]

How about converting it to a 6V6?
Pete

Hi Pete,

Thanks for the suggestion. Yeah, going with 6V6s would accomodate the plate voltages a lot better.....I'm just not sure if it is worth it to go to that much trouble......punching out the bigger holes in the chassis would have been a lot easier with a blank chassis, but I would have to disassemble this thing quite a bit. At some point in time, the effort and cost start to become on par with just building a superior-design amplifier.....

Thanks again.

[Paultergeist]

IIRC (real) 7189A were a higher dissipation version of el84s and should be OK at 380V, assuming they are still strong, and I bet the amp was designed that way and not out of wack. It's a pricey tube NOS. Problem is in the future or if someone swaps for fun or has a higher wall voltage than you.

Thanks for your input, rp.

This is a "friend-of-a-friend" type of situation, but I really want to bring this to the *right* outcome. A lot of reference data seems to point to 7189, 7189A, El84, etc tubes all being interchangable.......this is close, but it is not exactly true. There was a small re-wiring of the pins to allow for variations in internal connections (this I have already done)....and then there is the plate volateg issue. If I can bring the plate voltages down, then the amp should be safe to use any of the fore-mentioned tubes.

I did originally look into getting some actual 7189As, but as you noted, they are prohibitively expensive and quite hard to even find. Part of the issue, for me also, is trying to trouble-shoot this thing.....how can I eliminate the power tubes themselves as a possible factor if I don't have any to plug in to test? (Got plenty of spare EL84s around....)

[Paultergeist]

.....Supposedly there are new (JJs? The fat ones?) that are like 7189s and can take the higher voltage, others familiar can pitch in....

I have heard this as a rumor a couple of times.....but I have been unable to confirm it as of yet.

Does anyone know if JJ EL84s will take higher plate voltages?

[pops]

A good way to lower voltages is to put a large choke before the first filter cap, it will drop just about what you want David Funk talks about it is his book and i have done it, works great. Remember it has to be able to carry the current of the power tubes so has to be a larger choke, not so expensive as the diode and i think works better.

[labb]

I don't think you will have a problem running the EL 84's at 380 volt. Check and see what Peavey runs the EL84's in a Classic 50 at. I have measured B+ in one at 399 volt. Probably would need to re bias and check screen voltage.

[billyz]

The Sovtek el84m will work just fine as a replacement for the 7189, 7189a. They are not expensive. I just put a quad in a home
Stereo amp running well over 400vdc.
Or you could install the VVR circuit. In a cathode biased amp it is relatively simple. There are ways to limit the voltage either up or down and make
It fixed even.

[tubetek]

I also recommend the Russian EL84M; they will live in this setup...BUT-
Have You checked the filament voltage? I'll bet it's about 6.8-7.0 volts. My guess is that PT was spec'ed at 110VAC, common in the 60s. At nominal 120VAC these days (I'm 124 today), The best bet could be to lower the wall voltage. There's a cheap way of doing this floating around the web; uses a couple small Radio Shack transformers, if I recall...

[rp]

I don't think you will have a problem running the EL 84's at 380 volt. Check and see what Peavey runs the EL84's in a Classic 50 at. I have measured B+ in one at 399 volt. Probably would need to re bias and check screen voltage.

I agree, I don't think you need mods. I checked up and the EL84M look fine, maybe the fat TAD ones, not sure, maybe they are just fat and lazy and not tough and husky. Some don't seem to love the 'm's tone, whatever, it's a Teisco not a '64 Vox.

So you've got just plain modern EL84? Maybe when you get some 'm's and bias your V may drop bit and current won't be out of bounds, might sound good real hot? Too bad you don't have a stash to roll. Maybe Groove tubes w/ their number rating thing? That way the owner will always know what to get.

Schemo shows no screen grid resistors might want to add at least 100R. If your Vp and Vg2 are close then 1K-2.2K?

When done do check them in the dark just in case. I've had modern El84 glow even though nothing was very excessive.

http://www.tubesandmore.com/products/T-EL84-TAD
http://www.dougstubes.com/power-tubes/e ... l84-m.html

BTW bet old USA 6BQ5s would run very fine, they aren't too expensive ANOS.

[tubetek]

Here's what I was referring to... No mods to the amp, cheap AND portable...