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.
HT-40 Bias Information
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Last edited by sizzlingbadger on Thu May 26, 2011 4:06 am, edited 2 times in total.
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Good Info but you forgot mention connecting the speaker during the procedure.
Id hate for someone to mess up their amp
Id hate for someone to mess up their amp
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Fixed - thanks, knew I'd forget something
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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
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
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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.
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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
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
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Thanks a lot for this info!!! i end up replacing the power tubes with Mullard EL34's # 20 from the the tube depot (have a quad collecting dust from previos amps)sizzlingbadger wrote: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.
bias the amp to 55 mV with PR2. someone mention that PR1 may be to balance both sides of the push-pull circuit so no test point require for this........ maybe someone from blackstar can comfirm this.
any ways the amps is sounding excellent!!!!!!!!! i replaced temporarily the stock speaker with a WGS Veteran 30 (i am waiting for a scumback M75)
this amp is amazing for the price........
thanks again
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Superchampable wrote:Good Info but you forgot mention connecting the speaker during the procedure.
Id hate for someone to mess up their amp
Is this standard Bias procedure? The only other amp I've biased is an Egnater Renegade and it has the Bias test points and adjustment on the rear of the amp. Their manual specifically says to have speaker cab and jacks unplugged. Just curious.
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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-
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How different would this be for an HT-20?
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