Please comment on this EL84 PP design

[Captn Dave]

I picked up a nice little EL84 PP amp at a sale. It has a nice quad set of 6BQ5 and appears to be well constructed. It is a Scott; built by EH Scott's company Scott High Fidelity Laboratories in Chicago after his departure - Not HH Scott. I'd like your thoughts on the design which I'm concerned may make short work of the power tubes.

Here is the schematic

I brought it up on the variac and was a bit suprised to see the operating conditions for those power tubes. 330 volts on the plates and upwards of 40 ma on the cathode current. Over 13 watts - well over 100% power rating.

As you can see from the schematic, it is operating according to design specs. The design plate voltage is 330 vdc although my tube manuals all agree on a 300 volt max plate voltage. It calls for a 150 ohm shared cathode resistor. The actual value has drifted to 208 ohms but even with the higher value, the cathode currents exceeds 40 ma .

What are your thoughts on this. It seems too extreme to me. I'm thinking of installing an adjustable bias circuit and perhaps bringing down the plate voltage to 300. What do you think, am I being too much of an old lady or is this design a bit to hard on tubes?

[Fenris]

If these numbers are accurate, the tubes would be running a bit hot. I've seen EL84 designs with up to 360v on the B+, but only about 35ma of current per tube. Don't forget that the cathode resistor will cause a voltage drop of about 16 volts (40ma per tube, 208 ohms) giving a B+ of 314 volts, so your B+ isn't as high as it seems. Are you sure that's the current for each tube or both together? -16v on g1 should give alot lower current draw, assuming the tubes are close to spec. The original 150 ohms seemed about right - at B+ of 320v and g2 of 300v, it would give a combined current draw of about 72 ma (36ma per tube) and a g1 of -11v which works out according to the 1969 Phillips datasheet. I think you might be seeing the combined current draw, and it's low due to the increased resistance of the cathode resistor.

[Captn Dave]

Fenris,

I take your point about the plate voltage. Operating to spec it should be 330 less the 12 volts of cathode bias. 318 volts is clearly nothing to get excited about.

I did measure the cathode current as voltage drop across a 1R resistor in series with a single tube, not the pair.

I looked at the curve again and as you correctly note, the operating point is 318 and -12 volt bias (per the drawing), which on my drawing looks to be about 27ma plate current which would be about 8.5 watts power (which would be reasonable at 71% of the 12 watt rating).

Problem is, in my mind, that the two 27 ma curents across that 150R resistor is only going to create a 8.1 volt bias.

Now that I have gone through the thought process correctly, with your help, I don't see how you can have a paired 150R resistor, a 12 volt bias, and an EL84 operating at 318 v. Something has to give. It looks like 318v, 11.6v bias and a 193R resistor might get it.

I'll grant you that it does not look like these tubes are operating to spec. Perhaps I should look at again with another set.

[Rod Coleman]

EL84s are routinely run at 100% anode power rating, providing cathode bias is used. If you use a Sovtek/EH EL84, this is actually a Russian design, the 6P14P, which has an anode rating of 14W rather than 12W. These are tough valves, and I have pulled them from guitar amps that have seen years of duty at 13-14W and are still working and sounding right.

Running them too cold can degrade the overload sound, due to the 'blocking' effect rapidly underbiassing them at overload.

I often use 6P14P/EL84 with separate cathode resistors per valve if ratings are being leaned on - you don't want one of them taking 20% more power than the other if they aren't well matched. Use 300 or 330 Ohm for Sovteks depending on the B+.

BTW the Peavey Classic-50 runs them fixed bias at 410V or so and 10W, and doesn't have a track record of burning the valves up too badly.

[hollowstatefan]

I believe that this is internally the same as the Stromberg Carlson ASR-433, with different cosmetics. I think you can find schematics and servicing info online. I have a Scottsman without mag phono, the twin of the S-C ASR-333.

[Captn Dave]

Originally Posted by Rod Coleman

I often use 6P14P/EL84 with separate cathode resistors per valve if ratings are being leaned on - you don't want one of them taking 20% more power than the other if they aren't well matched.

I couldn't agree more. I have gone to separate cathode resistors on magnavox console pulls that use just one cathode resistor for the four power tubes with good results. It really helps to keep the current in line with non-matched tubes. I think I will use a 330R resistor with one of these 100R trim pots in series for an adjustable bias. Check them out, they are chassis mount and less than $3 each.

Originally Posted by hollowstatefan

I believe that this is internally the same as the Stromberg Carlson ASR-433, with different cosmetics. I think you can find schematics and servicing info online. I have a Scottsman without mag phono, the twin of the S-C ASR-333.

Good on you, Hollowstate! You are correct. I saw the photo in the post over on Audiokarma and it is the exact same chassis.

BTW- This amp came with a "Scott" tuner that was obviously HK, given that the entire tube compliment was HK. Scott must have given up on manufacturing their own equipment by the time these were made in 1959.

[Bas Horneman]

Thanks for that datasheet Captn Dave. Great trim pots.

[Captn Dave]

Originally Posted by Bas Horneman

Thanks for that datasheet Captn Dave. Great trim pots.

Yeah, aren't those dandy? Multiple turn pots for that low price and no mounting hassles one has with ordinary trim pots. At 3/4 watt they should be more than adequate for the job. A cheap way to add a little excellence to a fixed bias amp.

[Bas Horneman]

A cheap way to add a little excellence to a fixed bias amp.

Where do you order them?

[Captn Dave]

Mouser has them here in the US.