Blackstar HT-100 Head with runaway Bias... Schematic needed!

[pacAir]

I recently acquired a non-functioning Blackstar HT-100 head with no output tubes. Without the output tubes installed, all the switching and major power supply sub-systems seem to be just fine and the preamp produces a nice clean drive signal for the output tubes.

However, when the amp is turned on with new output tubes installed they go into "red-plate mode" almost instantly (several sets tried, just to confirm). The Standby switch doesn't disconnect B+, it forces the tube in and out of cutoff so you don't hear output but it is a fully functioning heat generator from the get-go.

I assumed the bias voltage at pin 5 of the EL34s should be in the ballpark of -50Vdc but I find +25vdc there! Problem identified!

I am sure I can fix this after an investment of multiple hours by going blindly through the circuitry looking for the problem but they use a different sort of bias control arrangement that appears to be a bias set/bias balance affair with 2 controls that do not appear to be left-right bias controls as they are interactive in a fashion that suggests the set/bias arrangement (based on my experience with the HT-40 variant that appeared to use a similar arrangement).

Since I do not know what the topology arrangement is without reverse-engineering it, I'd like to see if anyone has a schematic for this beast (or any one of its 40 or 60 watt relatives in the HT Venue series) that would give me the clues to this bias circuit arrangement.

Thanks for any help that may be forthcoming!


Steve

[pacAir]

Problem solved... after 4 hours of "amp wrestling" with no schematic.

I took up the challenge the amp was taunting me with and buckled down with multimeter, pen and pad. After reverse engineering the circuit (by measuring voltages and checking continuity on all circuit nodes within a half light-year of the bias adjustment circuits), I determined I was NOT looking for a failed component!

Every component I checked, disconnected or measured was fine on its own but the CIRCUIT wasn't right. I finally found the culprit! It was a combination of cheap resistors, rough handling during construction and an ill-advised circuit board layout choice.

One of the bias-injection resistors was mounted so tight against the board that the protective coating (paint) cracked at the point of contact with the PC board, which unfortunately was on the edge of a PC trace. The thin PC board mask layer didn't insulate against this resistor so a portion of the resistive element (Bias voltage of -96Vdc when in standby, -46Vdc when in operate mode) contacted the underlying trace (+480Vdc B+) and the contact point arced (almost 600Vdc potential at the point of contact in Standby) and welded the bias resistor tight to form an mid-component partial-short to a B+ trace!

This caused a low-impedance load on this high voltage node which heated up the dropping resistor effected so much that it was melting its own solder joint! This drain dropped the +480Vdc to about +56Vdc which was also back-flushed through the bias circuit creating output tube bias voltages of around +35Vdc instead of -45Vdc at pin 5!

This positive bias voltage caused the output tubes to red plate anytime power was applied and ultimately blew the B+ fuse. I took BOTH 33k bias-injection resistors and lifted them up off the board to prevent a recurrence of this problem and I put a dot of fingernail polish over the cracked PC board mask.

I'll plug into a cab and guitar tomorrow to see how it sounds as I am not convinced it is 100% yet. The digital reverb looks like it might be wacky (judging by the tremelo or chorus-like waveforms on the scope). The rest of the amp appears to be working normally though.

A few of my observational opinions regarding some design & construction choices:

I really don't care for the DB-9 footswitch connector... very low-tech. It is bulky, and NOT designed to be constantly inserted and removed and will DEFINITELY not stand up to the kinds of abuse these things are regularly subjected to. One of the screw sockets has already snapped off on mine, leaving the threads in the connector body. The whole connection scheme is amateurish and should have been better thought out. I think I may simply solder the switch end of the cable and completely eliminate the DB-9 nonsense at that end at least.

They could have also integrated the 4-buttons of the footswitch with better logic that would have expanded the functionality. Switch 1 could have been Clean/OD, Switch 2 could have been OD1/OD2, Switch 3 could have been Voice Select (that remembers the last voice selected on a per-channel basis) OR Effects-Loop bypass and Switch 4 with Reverb on/off as it is now.

The Egnater Renegade amps have a much more comprehensive 4-button footswitch that has a captive cable at the switch and 2 TRS connections at the amp end. I like this arrangement a lot better personally (assuming you can't do something like the Marshall "patented" method using only a single TRS).

This amp SCREAMS for bigger, better quality, more stable bias controls and dedicated bias measurement test points... BOTH accessible from the rear panel! The Vox AC50CP series and the Egnater Renegade series are a few I can think of that offer these features and they make life so much easier than having to open up the amp just the set the bias. These cheap trim pots on the board will change the bias if you blow on them (or vibrate them by playing the amp) and they were NOT locked down with a drop of fingernail polish or paint at the factory as they should have been.

I also found that this particular amp does not appear to meet power output spec. At 120Vac line it would only develop about 68 Wrms with EL34 output tubes. I put in a set of 6CA7 tubes and was able to get about 88 Wrms after biasing to about 34mA per tube... and NO red plates! Is this indicative of MORE problems or is it common with this model? Maybe this one was built on a Friday afternoon!


Steve

[Jazz P Bass]

Output Power

Most "100 Watt" amps that I test into a dummy load measure around 72-75 watts , clean output.
It is only when they are driven flat out, full bore, that the 100 watt figure is achieved.
I go for clean sine wave output figures.
Nice job on the bias repair!

[pacAir]

Honesty in Specifications. Does that term make you laugh or cry?

Thanks, it was a wrestling match all the way but I finally pinned it to the canvas!

I guess few people "honestly" spec amplifiers anymore and it is annoying! How else can an average user gauge tube wear or capacitor replacement time without waiting for an outright failure or a sudden major change in audio quality? How can anyone compare gear specs or match components from different manufacturers without some common ground in the method of deriving specifications?

About 6 months ago I repaired, re-capped and re-tubed a 1981 Music Man HD150 2x12 combo. When received it was putting out 78 watts and I KNEW it had problems. When I was done it was putting out 156 watts BEFORE clipping at 1kHz at less than 3 or 4 % distortion. It is becoming rare than anything made in the last 10 or 15 years even approaches what you are lead to believe is the output power rating. If they would would spec it as "100watts RMS @ 25% distortion" then I would expect 72 watts clean output power and quit bitchin' about it!

What ever happened to the fair-trade laws that prevented people from claiming "2000 Watts" of power without specifying the specifics on their 50 watt amp? Those inflated claims seemed to have started returning with car stereo gear in the late '80s, found its way into DJ equipment in the '90s and now has a foothold in every other field it seems.

I rank most Marketing people as just a hair better than most self-serving politicians and lawyers.


Steve

[Jazz P Bass]

Peavey has always been upfront when it comes to output specs.
A lot of times you have to read the specs real close.
900 watt powered mixer. Wow!
170 watts @ 8ohms/ per channel. Oh!
These newer boom boxes are a hoot.
300 watts output power!
The small label on the back states 23 watts pulled from the mains.
Magic or fiction?

[pacAir]

I plugged the amp into a speaker cab and found that it sounds quite good! I was impressed with the range of tones possible (at "home" levels anyway. I will be taking this head and a Marshall 4x12 cab to my regular jam tomorrow to see how it does in a live music full band situation.

I confirmed that the on-board digital Reverb IS working but sounds pretty digital at higher levels. I guess I need to tone it down with the switch and use it sparingly. Anyway, so far I'm happy with the outcome and excited to get it out in public and try it for real (sleazy footswitch connection and all)!


Steve

[pacAir]

I didn't get to play it live with a band for very long (only a few songs on rhythm. no leads!) because there were a lot of guitar players at the jam so I played more bass. However, I was liking what I heard from a reasonably short exposure to the amp.

One thing I neglected to add earlier. I was thinking I could pull two output tubes and turn this baby into a heavy-duty 50 watts head. NO CAN DO with this design! The four output tubes have their heaters wired in series and the quad heater string is fed from a single 25Vac winding on the power transformer! 25Vac into 4 series heaters = 6.25Vac per heater. Clever, but it limits your choices... not only can't you make the amp a 50 watter (with appropriate de-rating of the output impedance) but if you lose a heater in one tube you lose the ability to take 2 tubes out and finish the evening (albeit limping along). You have to have 4 tubes with good heaters! A small concern about something that almost never happens but there it is.

Steve