5F1 output tube

[jcool666]

I just finished construction on a tweed champ clone, with a few modifications. First, I used solid state diodes, with a 100 resistor before the 1st filter cap. 2nd difference is I bought a NOS 6CM6, which extensive research tells me is a direct equivalent to a 6V6, but in a 9 pin form.

Also, I have a quartet of used 7189s I salvaged from an old stereo receiver. I dont have a tube checker and since the stereo didnt work, I have no way of verifying the 7189s are good. So I wired the power tube socket on my champ for a el84/6bq5/7189. But when I fired up the amp, with an adjustable cathode resistor with a range of 100 to 1100 ohms, I couldn't get a plate dissipation of more than 4 or 5 watts out of either of the first 2 tubes I tried. I did check all the pins for continuity and didnt find any shorts.

My question is this: are the tubes bad, the circuit botched or is perhaps the plate voltage too high for the tubes? It measures about 395v plate to ground. I dont have much experience with the el84 type tubes, but what I've seen suggests they're really close to the 6V6. When I put a tone through, I got a decent sound out of the speaker, but I couldn't raise the dissipation despite dropping my bias resistance to 100 ohms.

[Stevem]

Tubes that are weak or shot will not pull much idle current or in turn dissipate much wattage.

With your near 400 volts on the plate and I would guess like 370 volts on the tubes screen, then if you install a 1 ohm 1% 1 watt resistor to ground at the end of the current Cathode resistor / pot that you have then if the output tube in question is good you should read atleast .040 VDC which will be the actual Milliamps of current the tube is ideling at.

In my blackface champ with a little over 400 volts on the plate and a 470 ohm Cathode resistor the amp idles at .045 amp, which is right there at the hot limit of bias.

Also keep in mind that a 6CM6 is only a 12 watt rated tube while a 6V6 is a 14 watt tube.

[jcool666]

It would be a real shame if these things are bad. I figured i could get a few bucks for a matched quartet of amperex 7189s.

I was planning on biasing the 6CM6 at around 10 or 11 watts. Running them a little cooler will give me earlier breakup. Also I put in an adjustable negative feedback, if I feel the need to clean it up.

[Stevem]

Feed back will only clean up the sound until the amp until it eats up that given amount of feedback voltage, then at that point your amp or any amp for that matter will jump right into hard on clipping pretty much.

To me and a ton of others a well designed amp that does not make use of feedback will be a much more playable amp if you need / like to play in that tipping point range!

The short coming is that with feedback the amp will have tighter low end responce until the clipping threshold gets reached.

[martin manning]

You're just messing with us here, I guess...

...I bought a NOS 6CM6, which extensive research tells me is a direct equivalent to a 6V6, but in a 9 pin form.

[pompeiisneaks]

You're just messing with us here, I guess...

...I bought a NOS 6CM6, which extensive research tells me is a direct equivalent to a 6V6, but in a 9 pin form.

Well I mean he may not have found it that easily... just sayin

~Phil

[martin manning]

All in good fun, of course!

[jcool666]

Well I mean he may not have found it that easily... just sayin

~Phil

I found it by combing through the 6V6 wikipedia page which listed a whole bunch of equivalents and near equivalents, and then checking to see which ones antique electronic supply carried. Reading the datasheet wasn't the exhaustive part. Doing anything on wikipedia tends to be pretty exhausting.

[Phil_S]

I am a little confused about what's going on with the power tubes. I'm not convinced they are bad. The 6V6 and it's equivalents (6AQ5, 6CM8) should have a 470Ω cathode resistor. EL84/6BQ5/7189 should have a 270Ω cathode resistor.

I suggest, stop trying to fiddle with the cathode resistor and use the recommended value for the tube. This amp is cathode biased. Sometimes cathode bias is called self-bias and there is a reason for this. As I understand it, as you change the cathode resistor, you shift the various operating points in a way that results in not much of a net change. You can probably read up on this and I'm not one to provide a technical explanation.

Assuming you have the recommended cathode resistor, it is easy enough to determine plate dissipation. Just measure the voltage drop across the cathode resistor (one probe on each side of the resistor, or one at the pin socket and the other to ground.) Then do the math. For a 6V6 or related, it should be around 19V. 19V/470Ω = 40.4mA. The general thinking is that 45mA is the maximum dissipation and that 90% of maximum is appropriate for a cathode biased amp. That would be around 40mA. If this is what you are seeing, then power out is around 4.5W and that is about right for a 5F1. All this assumes an output transformer with a primary of around 5KΩ.

Above all else, how does it sound to you? If it sounds good and the cathode voltage is around 19V, then I want to suggest you've got it working as it should.

[jcool666]

I was measuring across both the bias resistor and the OT primary winding. No matter where I had the bias pot set I couldn't get a reading above 15 mA. These are the 7189s that came out of an old HH Scott stereo. I haven't rewired the socket for the 6CM6 yet.

[martin manning]

Get all the pin voltages from the tube (cathode, g1, g2, and plate), and check the heater voltage while you're at it. Post them here, along with the cathode resistor value when the measurements were made, or the actual plate current measured using the OT primary resistance.

[Phil_S]

What Martin said, and while you are at it, remove the bias pot and insert a proper cathode resistor. For the 7189, use a 270Ω 5W. If you don't have 270Ω use 250Ω The pot could be bad and it isn't needed.

[jcool666]

Here's what I got:
G1:5mv
G2: 363v
A: 390v
C:4.16v

The heater is measuring @ 5.6vdc.

I tuned the bias to 270 ohms exactly.

Just for fun I also measured 3.73v across the 290 ohm primary winding.
Across the cathode resistor I calculated 15mA, across the OT I calculated about 13mA.

13mA @ 390v is 5w.

[martin manning]

5.6 vdc on the filament? I don't understand that. What's the VAC?

With those voltages, and only 13mA, it sure looks like that 7189 is toast if the filament is actually anywhere close to 6.3VAC.

[jcool666]

It's running on dc. I needed a supply for the led indicator and I had a regulator already built up.