I took delivery of some Winged "C" (SED) EL34 tubes today as backups and decided to test them out in my HT-40.

As you can see the physical size is slightly different to the stock TAD tubes which are a little shorter and fatter. The new tubes fit fine in the amp though.



Make sure you have a good quality multimeter and your test leads are in good condition. The voltages in the amp can kill you so be careful !

The Power & Standby switches must be ON and a 1/4" jack must be inserted into the input socket before you start. You must also have a speaker connected to the amp.

I measured the plate voltage and bias settings with the stock TAD's in place first. The plate voltage can be measured on both/either tube sockets between chassis ground and pin 3. The Bias can be measured between test points TP10 & TP18

Plate = 380V
Bias = 51mV

After changing the tubes and letting the amp warm up for a few minutes I took the plate and bias measurements again.

Plate = 379V
Bias = 54mV

As you can see there is very little difference and it was really not necessary to adjust the bias using PR2. If you use matched tubes you won't need to adjust the balance pot PR1.



These Winged "C" tubes have a pretty good track record for sound quality and they sound a little different to the TAD's, slightly more "woody" and "british". The TAD's sound good too in my opinion so I wouldn't change them out just for tone reasons. I really only did this to make sure my new tubes were ok and to post some useful info here.

Good Info but you forgot mention connecting the speaker during the procedure.
Id hate for someone to mess up their amp

Fixed - thanks, knew I'd forget something

I just got a HT 40, thanks for the info.

can i ask what number does your tubes have from the tube store? and how off were they from the 50 mv reading?

thanks for any help

They were both stamped gm 32. The mV bias reading changed from 51 to 54 when they were swapped. No change was required to biasing however I changed the bias to 60mV to warm them up a bit.

Thanks a lot! the reason i ask is because i bought this amp used and one of the power tubes was broken (the glass was broken) so i replaced the power tubes with Tungso EL43B # 34 from the tube store.

they sound extremely beautiful. i'll check the bias readings later on. to my taste they need to be a little bit hotter in this amp, the sound becomes fuller......

thanks

I replaced the TADs in my HT40 this evening as they were badly microphonic. Before I removed them I checked their bias voltage. They were 51.5mv and the glass is discolored 3/4" below the flash line. I replaced them with Tungsol EL-34B's matched at 25mv each. I checked the bias on them and was somewhat shocked to see they were at 32.4mv after letting them stabilize for 10 minutes. I raised them to 50.5mv with P2 and let them idle for 15 minutes and came back to see they'd stabilized at 50.2mv. I checked the plate voltage of each tube (chassis ground and pin 3 on each socket, separately). V3 was 403 DCV while V4 was 402 DCV. These tubes are rated for 400mv at the plate. The HT-40 doesn't allow you to adjust the plate voltage, only balance them, presumably with P1. I didn't see the need, just put everything back together. The microphonics are gone, no buzz nor feedback, just a very slight hum that goes away if I grab the cable plug, so a grounding problem. The clarity of the Tungsols was amazing out of the gate and remarked upon by my wife downstairs. Even turning up the master volume (gain channel still down at 9 o'clock) showed less increase in hum over the TADs, even when they were new. It's too late at night to turn the amp up and add any gain, so tomorrow I'll play with it. I did some before and after recordings with one of my patches (GT-10) and there was marked improvement in tone and clarity. It sounds like a different patch altogether! I'm eating dinner so not a proper writeup, any questions I'll be happy to reply. -Rod-

How different would this be for an HT-20?

Apparently according to Blackstar the HT20 uses a combination of cathode and fixed bias. See this thread....

http://blackstaramps.forumotion.com/t1405-ht20-combo-starting-a-biasing-fight

This being the case, I'm wondering how different biasing would be from the other amps in the HT line?
I've changed power tubes in mine without checking or adjusting bias, and it sounds fine, but now I'm thinking maybe I should give it a look.

Biased: I really look forward to seeing what you come up with. Thanks!

Marky -
They seem to make it difficult, don't they? Here's the EXACT line from the HT-20 Manual:

The carefully voiced preamp and
cathode biased dual EL34 power amp configuration provides the user with the ability
to create wonderfully lush valve tones.

In either case, by measuring a specific voltage across the R221 resistor, it sounds like you would still have to change a component (resistor) on the board to actually change the bias setting (Instead of adjusting a bias pot PR on the fixed bias 40). I need to open mine up again and see about the balance adjustment though.

Sometime today I'll get a seperate post up about what 3 sets of tubes (stock TAD, Sventlana and JJ) looked like in my 20 and compare plate voltage, ma, and dissipation in each pair. They were all well under a "hot" bias.

Thanks for the link!

AFAIK, the HT20 does need the bias setting up even though it is partly cathode biased. I haven't had mine apart yet, but I believe there are two trim pots the same as in the 40 (one for bias, one for balance), and you need to set the voltage across resistor R221 to 11.5V, then set the balance for minimum hum. As I said, I haven't done this yet myself, it's only what I've read, so please check before taking it as true.

Rob

You're right Rob, let me just open mine up and look - see if we can "handle the truth"

Ron