6v6 cathode bias question

[MattB930]

Hello all, long time reader-first time poster.

First, let me start by thanking everyone for their great help. I probably have an easy question but here goes.

I was given a few old Alamo Montclair and Electra amplifiers that are 40+ years old. My friend asked me to work on them. I've fixed the various hum, scratchy pot, normal wear and tear that an amp of that age would have.

However, my friend doesn't know how old the 6v6 power tubes are. I think they were purchased off the bay not working. They work now, however, there bias is completely off.

I hope I'm clear as mud here. Here goes.

Several of the amps have issues with very low volumes considering what they should be at. Let's use the Montclair Reverb as this example. Cathode bias. 2 6v6's, 1 12ax7 phase splitter, preamp section.

I measure the following:
Pin 3 (plate) = 236 v
Pin 8 (cathode) = 10.41 v
Cathode resistor = 190 ohms
Pin 4 (screen) = 210 v
Screen resistor = 15.38 kilohm's

Ok, so the math... 10.41/190 = 0.05479 A or 54.79mA
54.79mA / 2 = 27.395 mA per tube (shared cathode resistor)
0.027395 * 236 = 6.47 watts per tube

Is my math above correct?

Without power tubes I read 360 v on plate and 360v on screen.

I've tried another matched pair and get 5.95 watts based on similar math above.

I am correct that when a pair of power tubes are cathode bias'ed and they share a common cathode resistor you divide the mA by 2, correct?

Could this simply be because the tubes are so old?? They say... errr what is readable anyway say RCA on them.

Would a 6v6 really drop the plate voltage from 360v (with no tube) to 236v (with tube)?



Thanks for your help. If something is obvious or I am making a noob mistake, I apologize.

Matt

[Chuck H]

Ok, so the math... 10.41/190 = 0.05479 A or 54.79mA
54.79mA / 2 = 27.395 mA per tube (shared cathode resistor)
0.027395 * 236 = 6.47 watts per tube

Is my math above correct?

Yes. With one caveate. When cathode biased you should subtract the cathode voltage from the plate voltage for the math since this is the tubes "working" voltage. In cathode bias, instead of operating from a 0V reference the tube is operating from an elevated reference.

15.38k seems really REALLY high for a screen resistor. Did you check both resistors and do they read the same?

Could this simply be because the tubes are so old?? They say... errr what is readable anyway say RCA on them.

Would a 6v6 really drop the plate voltage from 360v (with no tube) to 236v (with tube)?

Yup. It's partly due to the resistance in the rectifier tube. The more current you run the greater the voltage drop will be too.

Not sure what other service you've been doing to these amps. I hope it has involved replacing the old electrolytic capacitors.

IMHO the example amp is biased too cold. You should decrease the cathode resistor and shoot for 35 to 40mA per tube. That won't make the amp much louder overall but should beef up the clean tone a bit.

These aren't loud amps. They use small OT's with lower efficiency and less fidelity, inefficient speakers, low plate voltages and tend to run the tubes very conservatively overall.

A tubes age won't make any difference to it's suitability, use will. If the tubes are old in use hours then they may be tired.

A couple of tips. Try a 5v4 rectifier (I'm assuming this amp has a 5y3 now). This will give you about 15 to 20V more plate voltage. And try a high efficiency speaker. A 3dB more efficient speaker will give you the same volume increase as doubling your watts. If you want to go further you could replace the OT with a better one. If you want to go all out you could replace both transformers, a higher voltage PT and a more efficient, higher fidelity OT. You can concievably get that amp as loud as a Deluxe or similar higher quality amp. Or you could do like I do. Get into good working order, crank it up and enjoy the creamy, elastic, not too loud distortion of these types of amps.

Chuck

[MattB930]

Thanks for the reply. One thing I forgot to point out is that it has a solid-state rectifier, no tube.

Ummm, let me double check the screen resistor when I get home. Now that you mention it I think the 15k is a value between the filter stages, not the screen itself.

One quick thing I did was change the 190 ohm resistor to one that measured 108 ohms. Doing so dropped the plate and cathode voltages while raising the current however this didn't change the power rating as I recall. Higher current but less plate voltage.

The main thing my friend wants is to get this amps usable for church and small gigs. He just wants them in good working order.

I do know that one of the amps, the electra, is running like 15 watts per 6v6 and it really sounds pretty sweet. The plate voltage on it is closer to 350v.

I just wasn't sure why two, somewhat the same - tube preamp/solid preamp stages, would be so completely different on the plates and so forth.

thanks for you feedback!

[MattB930]

Here are some pictures of the power tube/ filter section if interested.

I hosted this on my work site.. but if someone needs engineering work let me know... *shameless plug*

Amp Pictures

[Fred2202]

In most cases the power tubes will drop the voltage 10-20% compared to voltage with the power tubes removed. This is with readings taken at idle. How much drop depends upon how beefy the transformer is and the rectifier. To drop from 360v to 236v at idle means the power transformer is really undersized for the tube lineup or something else is wrong. Check the power resistors for drift or unusually high readings. The rectifier tube could also be weak. Swap it with a known good one if you can.

[Chuck H]

OK, so your not using a rectifier tube. To add to the confusion I ran the simulation with a dropping resistor AND a rectifier tube. In fact you should probably have about 320 to 330 volts with the tubes in. I can't imagine your PT is that grossly undersized so I'll ask again... Did you replace all the electrolytic caps in the amp? If these caps are failing they can cause all kinds of trouble like humming and excessive current draw. These caps have a working life of about fifteen years AND ACTUALLY GO BAD FASTER WHEN NOT IN USE. If you did not replaced them they are long over due.

As suggested you should check all resistor values for drift. With that plate voltage I can't imagine a 15k dropping resistor in the B+ rail between the plates and screens. 1.5k would seem more likely. So if that resistor has drifted so badly you should surely check all others.

Once you work out the voltage issue a good efficient speaker, bigger OT and proper bias will get the amp plenty loud for small gigs. If you need more oomph for medium gigs and a PA is available it's very easy to solder up a "line out" that runs off the speaker output. It can be plugged into a PA for a little extra kick and sounds pretty good. The bad news is that speakers and OT's are $$$ so it's really up to your friend how far he wants to take it. Ebay is a good source for frankenstien parts and speakers.