I have my doubts you will have 375v's, what is called for to drive drive that board, with the supplied 300V transformer. If you have not already installed it I would send it back and pay the difference for the AS-3TD360 - 360V Tube Amp Transformer. That is what is called for in the Dynaco build 360V. You may have the voltage necessary until you start biasing the output tubes then it is going to drop drastically. I did not find the 300V transformer up for the task.
ST-70 Tube Amplifier DIY Kit
- Thread starter wdecho
- Start date
Figures I should have known better than to buy inferiour goods I have many good amps and preamps, but I thought it would be a good lesson and teaching project at that low price. But I should have kept to my original gut feelings to buy known good gear that has problems or needs restoration and get them for a low price and use restoring them for the lessons and learning curve. Mark it down as a lesson learned.
It was a struggle for me to get this kit to work. Took me weeks but then I am old and admittedly my memory is no where as good as it use to be. But then in a way I enjoyed the struggle. Learned some things and now I have a working, good sounding PP amplifier. I would recommend beginners who want to build to just get on the waiting list at Dynakit if they want to build a Dynaco. There is no build guide, part selection or documentation with Antek's offerings. That said for the more experienced builder there is a cost saving.
A member on this thread was shocked to hear about 700V's on the plates of an EL-84 tube. Modjeski explained it this way. The plate is big, relative, in a tube and has no problem with a high voltage. It is the wires that will melt with too much voltage. The reason for 700V's is more volts to swing the audio signal in. He explained it this way. You start off with 700Vs. You will lose around 100V's when the tubes are under a load and be left with 600 V's to work with for the plus and minus of the audio signal to swing in, up and down from center point. If you start with say 400V's, which is common in a lot of amplifiers and lose 100V's under load you are left with 300V's to work with. Half as much to swing the signal in. With 700V's to start with you will have center point ,300V's for the plus of the signal and 300V's for the negative signal. Makes sense when explained this way. Roger Modjeski was a great teacher. Sorry he is gone. Very talented guy and his amplifiers still command a premium price. Music Reference was the name of his company.
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Ken I could very well be wrong about the supplied transformer that came with the kit. If you have already installed it give it a try. There could have been something wrong with what I did the first time I tried to build it. Dynaco does use a 360V transformer though. I hope you do not get discouraged. It is not a lot of trouble replacing the PS transformer and the one I bought cost $68. When I started looking for a replacement a Hammond is getting close to $300 these days. I just put the 300V transformer in the box of PS transformers I already have for future projects. Be glad to share my experience building this amp with you.
The plate element of most tubes can in fact operate at much higher voltages than published ratings indicate. Often, the limiting factor in plate voltage rating was as much a factor of the insulating materials available back in the day for tube bases and sockets as any anything else -- that was the primary reason that some power tubes used a plate cap, which then allowed for a higher plate voltage rating at the time. The 807 with its plate cap is a perfect example of this which otherwise uses the same internal structure as the 6L6.
With the 6BQ5 family of tubes having no base, and using ceramic sockets, the plate voltage can be elevated significantly. But, you never get something for nothing: The down side of elevating the plate voltage so significantly is that the reflected impedance from the output transformer also needs to be significantly elevated as well, creating real design issues for the output transformer if a flat power response from 20Hz to 20 kHz is to be achieved. The low end isn't the problem, but at the high end, the elevated winding capacitance due to the greater turns ratio required is.
Glad you got your amplifier sorted out!
Dave
With the 6BQ5 family of tubes having no base, and using ceramic sockets, the plate voltage can be elevated significantly. But, you never get something for nothing: The down side of elevating the plate voltage so significantly is that the reflected impedance from the output transformer also needs to be significantly elevated as well, creating real design issues for the output transformer if a flat power response from 20Hz to 20 kHz is to be achieved. The low end isn't the problem, but at the high end, the elevated winding capacitance due to the greater turns ratio required is.
Glad you got your amplifier sorted out!
Dave
I'm far less concerned about the plate and it's ability to handle voltage. Voltage in and of itself is meaningless, its the potential difference between things that matters. I'd just be worried that the plate and suppressor are awful close together in a small tube like an EL84, and that seems like a good way to turn it into an arc discharge lamp.
Modjeski mentions increasing the primary impedance of the OPT when raising the voltage at the BAF 2015. Glad you added this.
There is no current flowing in the suppressor grid so I do not see a problem there.
Scary isn't it but Roger Modjeski pulled it off with the Music Reference RM10 and got an impressive 35 watts per channel from the EL-84 tube when most PP El-84 amplifiers were trying to achieve half that much, 17.5 watts. I started a thread this morning with a link to his presentation at the Burning Amp Festival 2015 if you have an interest on how he designed and did it. Personally working with that kind of voltage is scary to me. I have been bit with 400V's and it hurts. Not only just the initial shock but my finger felt the effect for much longer. You always hear disclaimers cautioning us about working on tube amplifiers and rightly so but we are talking about DC mostly. Shock the crap out of you but more like to get hurt getting away from it. Be careful but I have not heard of anyone dying getting shocked working on electronics lately or at all. Very rare.
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I said nothing about current in the suppressor grid. I said there is a voltage difference. 700v on the plate, ground on the suppressor. Get those two things too close together and suddenly there is a whole lot of current flow between them in the form of an arc. That much voltage on that small of a tube makes me uncomfortable. Y'all can do what you want, but I'll pass on that.
I do find the fact that you mention this was talked about at the Burning Amp festival to perhaps be telling, since thats exactly my concern.
As far as I remember he did not mention any concern of the suppressor grid with 700V's on the plates at the BAF. What I was referring to was an increase of impedance needed on the primary of the OPT's if using, I believe the figure was 710V's, on the plates being mentioned by another poster. Again if memory is correct he was using 12K impedance OPT's. He wound his on transformers for his amplifiers. Obviously it can be done due to the RM10 being a success and still command a premium price on the used market.
Like yourself though, not something I would want to try. Mighty small tube.