[ttan98]

Roger,

I have been fascinated and an admirer of your RM200, 100wats/channel amplifier. I would
like to try to design an amplifier similar to RM200 but NOT a clone. I have a topology in
mind I would appreciate if you can answer a few questions I currently have in mind which I
need your assistance. Your answers do not have to refer to the design details from RM200
but a design you think will work well.

I have some experience with design, construct and installation of valve amplifiers, I have
built many units in the past. I like to place a few design questions here and if you decide
to help me I will keep the questions short and succinct and will also keep the follow up to
your answers to a minimum. I envisage I will have about  4-5 questions.

Please let me your decision. A favorable response is most welcomed.

[ttan98]

Hi,

I forget to include one important fact, I am building this amp for myself and for a friend. I am not interested in marketing it commercially. BTW I live in Australia.

[Roger A. Modjeski]

Gooday,

sure ask away

[ttan98]

Roger,

Many thanks for a positive response to my request. My questions are not so much asking your
response to my detailed design but rather general questions to ensure a design topology
that will work well, i.e. design and build  a 100W amp. at low distortion and reasonable amount of Damping Factor.

1. I agree with you that to design a reliable and long lasting amp, it should be bias at
low current and KT88/KT120 is strapped in Pentode mode, at anode set at around 700Vdc and
screen at 320Vdc. KT88 in Pentode mode has high output impedance, how would you reduce it? 
From what I know from Tube design a shunt feedback on the output tube, ie KT88 would reduce
it would you recommend this design if not why not? Your opinion please.

2. You have achieved a damping factor of greater than ten from your RM200, it is by no
means easy from a tube amplifier, did you achieve this by using large loop gain that is why
you use transistor in the input. If so what loop gain can you apply to a tube amplifier at
the same time maintain unconditional stability? Is it through special circuitry at the
output stage or anywhere else? Any design suggestions to achieve DF=10?

3. The output transformer is of prime importance, obviously I cannot buy your output
transformer, however there are a no. of commercial o/p transformer suppliers such as
Hammond, Edcor, Magnequest, and a few others, which supplier should I source my o/p
transformers from, can you even name a model which I should buy, i.e. to attain
100W/channel the anode to anode impedance should be 5Kohm and at the same achieve low
distortion. Is this correct?

A definitive answer to these questions will help  me reduce the design time in experimenting
with various design options. Those who read this would like to contribute are most
welcomed. I may have another 1-2 questions left, but still formulating them at the back of my
mind.

Again thanks awaiting your response.

[Roger A. Modjeski]

Roger,

Many thanks for a positive response to my request. My questions are not so much asking your
response to my detailed design but rather general questions to ensure a design topology
that will work well, i.e. design and build  a 100W amp. at low distortion and reasonable amount of Damping Factor.

1. I agree with you that to design a reliable and long lasting amp, it should be bias at
low current and KT88/KT120 is strapped in Pentode mode, at anode set at around 700Vdc and
screen at 320Vdc. KT88 in Pentode mode has high output impedance, how would you reduce it? 
From what I know from Tube design a shunt feedback on the output tube, ie KT88 would reduce
it would you recommend this design if not why not? Your opinion please.

Not sure what you mean by shunt feedback?

2. You have achieved a damping factor of greater than ten from your RM200, it is by no
means easy from a tube amplifier, did you achieve this by using large loop gain that is why
you use transistor in the input. If so what loop gain can you apply to a tube amplifier at
the same time maintain unconditional stability? Is it through special circuitry at the
output stage or anywhere else? Any design suggestions to achieve DF=10?

I use a matched pair transistor at the input to achieve exceptional common mode rejection of over 100 dB (rejection of cable noise). Rather than look at the loop gain, just look at the final performance under various loads. The feedback in most amplifiers is typically is 10-20 dB to give you a range.

3. The output transformer is of prime importance, obviously I cannot buy your output
transformer, however there are a no. of commercial o/p transformer suppliers such as
Hammond, Edcor, Magnequest, and a few others, which supplier should I source my o/p
transformers from, can you even name a model which I should buy, i.e. to attain
100W/channel the anode to anode impedance should be 5Kohm and at the same achieve low
distortion. Is this correct?

Actually you can buy any of my transformers and we can make special ones too. I don't recommend any from your list. The high plate voltage is out of their realm.

A definitive answer to these questions will help  me reduce the design time in experimenting
with various design options. Those who read this would like to contribute are most
welcomed. I may have another 1-2 questions left, but still formulating them at the back of my
mind.

The RM-200 is not like any other (typical) power amp and it took a great deal of time to develop it. As you may know I am more interested in making things that have not been made before than just making amplifiers. There are too many variables to deal with for the average builder.

Since you already have the voltage parameters why not take a simple driver circuit like the Marantz 8B. I recently worked on an amplifier that was built at the Randall Museum (classes given in San Francisco) and with some tweaking it became very nice. Like the Marantz it has a single triode input stage followed by a long tailed pair phase inverter. A classic circuit which performs well, is easy to manipulate and make stable. You will need some test equipment to do these experiments and that is where the fun begins. You are just the kind of person I am looking for in my school where we intend to build really great stuff. I have all the test equipment and am willing to share what I know with dedicated students.  When you consider what amplifiers cost these days a trip to the USA might be a bargain. 

However if this is your first amplifier project I would advise you to start with lower voltages for your own safety and the safety of the tubes. At 700 Volts things happen very fast and furious. What is your level of experience?

Again thanks awaiting your response.

[Roger A. Modjeski]

Here is a good discussion of loop gain. This is complex stuff. Start with the first video and go as far as you can.

https://www.youtube.com/watch?v=oOsltCa7qBo

[ttan98]

Hi,

Thanks for the response, I am not sure it helps me that much. I have a degree in Electrical engineering I understand tube design. Anyway thanks for your effort and I can continue the work myself.

[Roger A. Modjeski]

What else do you want to know?