The second schematic you listed Hashimoto KT-88, the HV power supply section is incorrect, as shown.
Art
Art
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What's wrong with the design and is it easy to fix? Is the el34 design correct? I'm not really good with electronics design but I can solder.
PS - I am aware of the voltages and dangers and will have help building this but first I must settle on a design.
Thanks
PS - I am aware of the voltages and dangers and will have help building this but first I must settle on a design.
Thanks
This KT-88 schematic reaches 2% of THD at about 7W of power.
Do you really want it?
Hashimoto schematic:
Too much trust in the wiper of the balance pot.
If ever loses contact, runaway bias is assured...
Fairly simple rearrangement would assure that
loss of contact gives maximum negative bias.
--
Also heater of splitter stage is 194V below the
cathode at 6.3VAC negative peak. If everything
is working exactly right, this is just barely safe.
And does not account the cathode voltage will
swing 1/2 any input seen by this stage. Then
perhaps is not safe. Heater to cathode negative
200V max DC+Peak according to GE.
Isn't heater bias ideally supposed to be same
or slightly positive of the cathode?
Too much trust in the wiper of the balance pot.
If ever loses contact, runaway bias is assured...
Fairly simple rearrangement would assure that
loss of contact gives maximum negative bias.
--
Also heater of splitter stage is 194V below the
cathode at 6.3VAC negative peak. If everything
is working exactly right, this is just barely safe.
And does not account the cathode voltage will
swing 1/2 any input seen by this stage. Then
perhaps is not safe. Heater to cathode negative
200V max DC+Peak according to GE.
Isn't heater bias ideally supposed to be same
or slightly positive of the cathode?
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Still looking
Thank you everyone. I will keep looking/receiving recommendations for 50+ watt tube amps then.
I have another question regarding the Dynaco mkiii's. If I were to have an output transformer custom built of very high quality, a poseidon board, along with a sds power supply. Would this be a very good design (same league as a mc75/citation) or still compromised (like dropping a Ferrari engine into a Honda Accord)?
I would prefer to not go this route as the design from what I have read runs the tubes quite hard.
Any opinions on torroidial output transformers and this Plitron Vanderveen design? Tube Amplifier Circuit Board | Plitron
Thank you everyone. I will keep looking/receiving recommendations for 50+ watt tube amps then.
I have another question regarding the Dynaco mkiii's. If I were to have an output transformer custom built of very high quality, a poseidon board, along with a sds power supply. Would this be a very good design (same league as a mc75/citation) or still compromised (like dropping a Ferrari engine into a Honda Accord)?
I would prefer to not go this route as the design from what I have read runs the tubes quite hard.
Any opinions on torroidial output transformers and this Plitron Vanderveen design? Tube Amplifier Circuit Board | Plitron
Ferrari? Nah, more like dropping an Acura engine...🙂 Speaking of Plitron/Van der Veen, have you check out this fully documented design, it should meet your needs.
Jazbo8, thanks for that link! Just came in handy for my PP EL-152 built.
Looks superb and must've cost a bomb to own one!
Just print out the 11 pages to have a close scrutiny of the figures.😀
Most of the costs are in the Plitron OPT's and the pretty chassis - you get what you pay for...😉
Wow... Plitron OPT! Just email Menno Vanderveen for advice for a PP EL-152 built.
Before all this, I was going thru all the PP GU50 schematics for guildance.
Hey Pegasus123...have to thank you because thanks to your post discovered the big red board by Pete Millett that uses cheap television tubes and with a little mods is able to arrive near 100WPC ...even for a novice... and for me the opinion of so much people that I admire (Pete, Mr. Tubelab..) is OK to go and try.
The red board is on the way, just have to understand some details where hope to receive help from experts, if you don't want to read all 149 pages of the thread here's a link with a novice resume :
http://www.diyaudio.com/forums/tubes-valves/151206-posted-new-p-p-power-amp-design-149.html
Don't know if this would be consistent with your search.....
Thanks and best luck
giuseppe(madrid)
I mentioned that one back in post #12. It is a great sounding amp with big power capability. I saw your question in that thread about how to get 70WPC.
I will need to read through the thread again myself since it's been a while to find some of the details, maybe tonight. Pete designed the board to operate on about 350 volts and produce about 20 WPC.
An OPT swap alone will get you around 40 WPC.
An OPT swap and a power transformer swap gets more power but I don't remember the numbers.
An OPT + power transformer + tube swap gets more, but I don't remember if you can get to 70 WPC this way.
The limit for the power supply on the board as about 450 volts (with 500 volt caps). I believe this gets you in the 70 WPC range.
To get 100 WPC and beyond requires an extra power transformer. In all cases I used the 6GU5 as the driver tube. It is a hexode and has the most gain VS distortion of everything I tried in this board. The stock tubes are fine at 20 WPC, but more gain is needed in the first stage to get to 100 WPC+.
Plitron OPT's are very good transformers. I have a pair that I use for reference standard testing, and I just got a second pair for the "big one" that I plan to build. It will be in the 400 WPC range.
It is my opinion that the amp design shown in the schematic for the PC board is suitable for a much lower powered amp. In fact it is very similar to my SPP amp board which used 2 X EL84 per channel for about 15 WPC.
For bigger amps prefer a fully balanced design like Petes, or the other design I linked to in post # 12. That is the topology that will be used in the 400 WPC amp. Pete's board design doesn't quite have enough gain to go beyond 100 WPC.
[tubelab.com]I mentioned that one back in post #12.
That's were I found the link you posted, and spent last week reading and reading.🙂
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[tubelab.com]The limit for the power supply on the board as about 450 volts (with 500 volt caps). I believe this gets you in the 70 WPC range.
That's what I want, around 70WPC and minimum changes to stock.
To have 450V on the board, how should choose the power transformer?
Would it be OK a 350-0-350 (for example) or how do calculate?
------------
[tubelab.com]In all cases I used the 6GU5 as the driver tube. It is a hexode and has the most gain VS distortion of everything I tried in this board. The stock tubes are fine at 20 WPC, but more gain is needed in the first stage to get to 100 WPC+.
OK but to use 6GU5 as driver instead of 6CB6 are changes needed?
Thanks
That's were I found the link you posted, and spent last week reading and reading.🙂
------------
[tubelab.com]The limit for the power supply on the board as about 450 volts (with 500 volt caps). I believe this gets you in the 70 WPC range.
That's what I want, around 70WPC and minimum changes to stock.
To have 450V on the board, how should choose the power transformer?
Would it be OK a 350-0-350 (for example) or how do calculate?
------------
[tubelab.com]In all cases I used the 6GU5 as the driver tube. It is a hexode and has the most gain VS distortion of everything I tried in this board. The stock tubes are fine at 20 WPC, but more gain is needed in the first stage to get to 100 WPC+.
OK but to use 6GU5 as driver instead of 6CB6 are changes needed?
Thanks
you should search for the new dynaco kits, one would be hard pressed to build a better amp at a lower cost
Another vote for Pete Millet's big red PCB, it's miles ahead of the original Dynaco design and most of the after-market boards.
I built a pair of these monoblocks, the 100 Watt version not the 70 Watt, running a quad of EL34 on each. Proper build using the recommended Plitron Power and Output Trannies, the Plitron/Vanderveen PCB and Blackgate cathode bypass caps on the front end etc.
Probably sufficient to say they are sitting on my workshop shelf not being used.
In Pentode Mode they have an output impedance so high as to be unusable (except maybe for 16 Ohm Speakers).
In Ultralinear Mode they suffer parasitic oscillation in the output tubes unless you up the screen grid resistors to 1K and output impedance is still high (but usable).
With the Output Tubes in triode mode they were "nice" but a bit "bland".
Swapping an ECC99 for the 12AU7 is a definite improvement.
They currently have ECC99 in the front end with the EL34s in triode mode. I get a bit over 40 Watt per channel just below clipping.
I drag them out from time to time and have another listen but am always happy to revert to my 10W per channel Baby Huey.
I plan (one of these days) to rebuild them completely into a parallel Push Pull 300B which will actually mean a new build using just the power and output trannies, unless I can find some mug (err.. I mean some enthusiast) who wants to buy them from me.
So my advise is to go with one of the other recommendations above.
Cheers,
Ian
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Jazbo8,
I should be "fair" here. I did'nt like them much but at least one other person who is a regular listener to my system loved them.
I have a theory/opinion/guess which is as follows:
A large lump of that "traditional" tube amp sound is increased 2nd Harmonic distortion at low frequencies as the impedance from the primary inductance falls toward, and even below the effective rp of the output tubes. This was written about by Mr Partridge and other guru's of his vintage. The torroidal output trannies have a primary inductance of nearly X10 most EI trannies. Bass therefore goes a lot lower but is subjectively weaker.
As a check of this I compared the PAT4006 torroidal trannies against Hammond 1650T in a prototype 4 x KT88 amp with a Allen Wright inspired cascode diffamp front end. The Hammonds have a primary inductance in the 50-60H range, the PAT 4006 have a primary inductance of >500H.
I liked the subjectively stronger bass from the increased 2H distortion at low frequencies afforded by the Hammonds but to say that the amp was "better" with the Hammonds would be technically incorrect.
Cheers,
Ian
I should be "fair" here. I did'nt like them much but at least one other person who is a regular listener to my system loved them.
I have a theory/opinion/guess which is as follows:
A large lump of that "traditional" tube amp sound is increased 2nd Harmonic distortion at low frequencies as the impedance from the primary inductance falls toward, and even below the effective rp of the output tubes. This was written about by Mr Partridge and other guru's of his vintage. The torroidal output trannies have a primary inductance of nearly X10 most EI trannies. Bass therefore goes a lot lower but is subjectively weaker.
As a check of this I compared the PAT4006 torroidal trannies against Hammond 1650T in a prototype 4 x KT88 amp with a Allen Wright inspired cascode diffamp front end. The Hammonds have a primary inductance in the 50-60H range, the PAT 4006 have a primary inductance of >500H.
I liked the subjectively stronger bass from the increased 2H distortion at low frequencies afforded by the Hammonds but to say that the amp was "better" with the Hammonds would be technically incorrect.
Cheers,
Ian
That's a good theory, the bump at LF is gone with the Plitron OPT. Perhaps this is also the reason why many people like to use the "loudness" button on their stereo, especially at low listening levels. That begs the question, would the Plitron really shine when playing at high output levels, assuming you have a large enough listening space and some speakers that can reproduce the lows.
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