[corndog71]

It's interesting to me how many tube designs I've seen where the transformers are fairly close to the power tubes.  The old Dynaco MkIII put 2 big transformers on a 9"x9" frame next to 2 big power tubes with perhaps a little bit of space for the driver tube.  Based on these old amp's success I would guess there isn't much interaction.  Still I know how sensitive line level signals can be to power supplies and wonder if spacing things out is worth it.

When building my kit amps I opted for a bigger enclosure than originally called for partly because of the larger caps I'm using but also to spread everything out a bit.  The most obvious benefit is a much cooler amp.  I guess what I wanted to ask was just how much do these elements interact and what are some of the best ways to minimize interference.

[jupiterboy]

Seems like potted transformers with Mu metal cases might be an interesting solution.

[Roger A. Modjeski]

Its the power transformer you need to be concerned about as it will radiate flux into various parts of the circuit. Put it as far away as you can and of course turn it 90 degrees from the output transformer as you see in the Dynaco MK III. Being close to the tubes is not a worry.

[corndog71]

Would having driver tubes close to power tubes cause any problems?

[Roger A. Modjeski]

Yes, because the output tubes are swinging a lot of voltage on the plates the capacitive coupling to the driver tube could cause problems in making the amp stable at high frequencies. The coupling can cause the response to rise or fall depending on if it is positive or negative.  I sincerely wish you could take my amplifier classes as these are questions of a high level.

[corndog71]

I would love to!  I don't suppose you would be interested in moving to Chicago?  There are no massive fires here although it does get a bit chilly for about 8.5 months out of the year.

I remember reading some old KT88 data sheets that a 4" center to center spacing was recommended.  What about EL34 tubes?  I'm planning on scratch building a ST70 but wanted a different layout than the classic dynaco chassis.  Kinda like what I did with my VTA-ST120 but not exactly. 

Here's a sketch I made of my idea.  I don't have my measurements handy but I think it came to 2 1/4" spacing center to center between tubes which only gives about 1 1/2" of space between the actual glass.
My box is 17"wx13"dx2"h.



This was another alternative layout.  Driver tubes would be set back 1"-1 1/2".


I'm limited by the Hammond boxes available.  I really only want 2" of box height and that leaves me with a 17x13" box.  Of course I could use Front Panel Express and just make a plate to mount everything to while doing something similar to your wood frames.  I don't want to make too many copies of other's designs. 

[Roger A. Modjeski]

The reason for 4 inch spacing of output tubes is just for cooling and I recommend it.  Putting the drive in between won't change that much. With 17 inches you have enough room.

The second sketch looks better.

My parents were from Chicago and I have been there many times as a kid and later as an adult at CES. Its not my kind of weather but is a nice city. Is there much audio interest there? I could give a class if there was.

What is you circuit going to be? What performance are you hoping to achieve?

[corndog71]

The reason for 4 inch spacing of output tubes is just for cooling and I recommend it.  Putting the drive in between won't change that much. With 17 inches you have enough room.

The second sketch looks better.

My parents were from Chicago and I have been there many times as a kid and later as an adult at CES. Its not my kind of weather but is a nice city. Is there much audio interest there? I could give a class if there was.

What is you circuit going to be? What performance are you hoping to achieve?

It's a big bustling city so I would think there would be some interest.  We have a handful of audio shops that have been in business for years and have even gotten bigger in that time.  Record Store Day draws a surprising number of people and seems to be growing each year.  Lots of of younger kids getting into vinyl.  Not like it used to be obviously but still promising.

As for the amp, I guess I'm just itching to build another one.  While I've been building tube gear and studying it for the last 17 years, It's mostly been a part-time hobby of mine.  The last few years I finally challenged myself to learn more about tube amps and to attempt to build them using various circuit ideas.  My first attempt combined features that nobody else offered.  So I built it myself.  It worked so well I built another one and added my own tweaks to it.

So now I have a 17 watt amp and a 60 watt amp, and those were so much fun I want to build another.  The ST70 parts and circuit is readily available and the Vacuum Tube Audio driver is a winner so I want to use that along with Clarity Cap TC series power supply caps in order to make it a little more unique.  Yeah, circuit-wise it's nothing new but I'm not sure I'm at the level to explore new ideas without a little more education.  I appreciate everything you're willing to share. 

[Roger A. Modjeski]

It's a big bustling city so I would think there would be some interest.  We have a handful of audio shops that have been in business for years and have even gotten bigger in that time.  Record Store Day draws a surprising number of people and seems to be growing each year.  Lots of of younger kids getting into vinyl.  Not like it used to be obviously but still promising.

As for the amp, I guess I'm just itching to build another one.  While I've been building tube gear and studying it for the last 17 years, It's mostly been a part-time hobby of mine.  The last few years I finally challenged myself to learn more about tube amps and to attempt to build them using various circuit ideas.  My first attempt combined features that nobody else offered.  So I built it myself.  It worked so well I built another one and added my own tweaks to it.

So now I have a 17 watt amp and a 60 watt amp, and those were so much fun I want to build another.  The ST70 parts and circuit is readily available and the Vacuum Tube Audio driver is a winner so I want to use that along with Clarity Cap TC series power supply caps in order to make it a little more unique.  Yeah, circuit-wise it's nothing new but I'm not sure I'm at the level to explore new ideas without a little more education.  I appreciate everything you're willing to share.

What sort of test equipment do you have? What is the "vacuum tube audio driver" you refer to above. We don't need any more ST-70s.

[corndog71]

Not much.  I have a nice fluke multimeter.  I'm hoping to get an oscilloscope someday.  Do you have any recommendations? 

I take it you're sick of the dynacos. 

[Roger A. Modjeski]

Not much.  I have a nice fluke multimeter.  I'm hoping to get an oscilloscope someday.  Do you have any recommendations? 

I take it you're sick of the dynacos.

For $99 in its day the Stereo 70 was a nice amp. However the driver is very sensitive to tube parameters and can have a lot of distortion if the 7199 doesn't bias up right. It is hard on output tubes, but you can turn down the idle current. Then the b+ goes up and may blow the filter.  The power transformer runs hot and often fails. The output transformers are ok.

We do sell a 6GH8 adapter to replace the 7199 and we test the tube to bias up right.

[corndog71]

For $99 in its day the Stereo 70 was a nice amp. However the driver is very sensitive to tube parameters and can have a lot of distortion if the 7199 doesn't bias up right. It is hard on output tubes, but you can turn down the idle current. Then the b+ goes up and may blow the filter.  The power transformer runs hot and often fails. The output transformers are ok.

We do sell a 6GH8 adapter to replace the 7199 and we test the tube to bias up right.

I think I heard the original driver circuit once before I got into tube gear.  I was not impressed at the time.

This is the driver circuit I was referring to. 
http://www.tubes4hifi.com/ST70.htm

It sounds a lot better.

[Roger A. Modjeski]

I think I heard the original driver circuit once before I got into tube gear.  I was not impressed at the time.

This is the driver circuit I was referring to. 
http://www.tubes4hifi.com/ST70.htm

It sounds a lot better.

do you have some links to schematics?

[corndog71]

They don't publish their schematic but describe the circuit as using half of a 12BH7 for voltage gain and another 12BH7 per channel for phase splitter/ driver in a long-tailed pair configuration.  13 dB of feedback is used.  There is a pot per output tube for adjusting idle current.  Also a constant current source is used to maintain AC balance.  A beefed up power transformer and choke is supplied with the kit as is additional capacitance.

They like to retain the original dynaco chassis and look so one could upgrade their vintage amp if desired.  Personally, I find the ST70 look dated which is why I wanted to change the layout and appearance.  Did you happen to look up the caps I mentioned? 
http://www.claritycap.co.uk/products/tc.php

I found them to sound better than electrolytics and especially than multi-cap can.  Yes, they cost more but the improvement in sound and long-term reliability is in my opinion worth it.

[Roger A. Modjeski]

They don't publish their schematic but describe the circuit as using half of a 12BH7 for voltage gain and another 12BH7 per channel for phase splitter/ driver in a long-tailed pair configuration.  13 dB of feedback is used.  There is a pot per output tube for adjusting idle current.  Also a constant current source is used to maintain AC balance.  A beefed up power transformer and choke is supplied with the kit as is additional capacitance.

They like to retain the original dynaco chassis and look so one could upgrade their vintage amp if desired.  Personally, I find the ST70 look dated which is why I wanted to change the layout and appearance.  Did you happen to look up the caps I mentioned? 
http://www.claritycap.co.uk/products/tc.php

I found them to sound better than electrolytics and especially than multi-cap can.  Yes, they cost more but the improvement in sound and long-term reliability is in my opinion worth it.

On the subject of electrolytics: Making long life electrolytics takes many years of experience. I use a lot of Nichicon because they seem to last forever. We have RM-9s out there that are 30 years old now and the filter caps are still in excellent condition. The same is true for the RM-4 and RM-5 preamps.  I am wary of a new company or a new cap design that has not been tested over time. Do we know how long these have been around?

How are the SA series coupling caps?  Are they well liked by the general public in the forums?

I am looking for some well liked caps for a factory upgrade option in my amps. Problem is there are so many. On special request I do install the purchaser's favorite caps whatever they may be. Some people like to spend a lot of money on caps. Problem is that I cannot stock the wide variety of caps that are appearing on the market. Also I have seen "audiophile" caps either open or short. The failure rate of these caps is much higher than from an experienced maker like Nichicon or RelCap. I cannot warranty the damage done if a cap of the purchasers choice fails. I do warranty the amps if my chosen cap fails.

[corndog71]

On the subject of electrolytics: Making long life electrolytics takes many years of experience. I use a lot of Nichicon because they seem to last forever. We have RM-9s out there that are 30 years old now and the filter caps are still in excellent condition. The same is true for the RM-4 and RM-5 preamps.  I am wary of a new company or a new cap design that has not been tested over time. Do we know how long these have been around?

How are the SA series coupling caps?  Are they well liked by the general public in the forums?

I am looking for some well liked caps for a factory upgrade option in my amps. Problem is there are so many. On special request I do install the purchaser's favorite caps whatever they may be. Some people like to spend a lot of money on caps. Problem is that I cannot stock the wide variety of caps that are appearing on the market. Also I have seen "audiophile" caps either open or short. The failure rate of these caps is much higher than from an experienced maker like Nichicon or RelCap. I cannot warranty the damage done if a cap of the purchasers choice fails. I do warranty the amps if my chosen cap fails.

I haven't tried the SA series.  Others have compared them to Solen.  I've tried the ESA series and they give a really detailed yet warm midrange however I find the upper treble a bit dark sounding.  The MR series are excellent top to bottom but are a bit expensive. 

The biggest downside to the TC series are their size.  But they have an extremely low ESR.  When cutting off my amp they drop most of their stored energy very quickly.  The TC caps have only been around a few years so I can't say for certain how long they last.  On the outside they appear very well made.

When I built my first amp I used Panasonic electrolytics.  The amp sounded great for months.  Later I was curious about the TC caps and decided to try them out.  The first thing I noticed was that it seemed like a haze had been lifted off of the sound.  That's the best way I can describe it.  Both caps had great dynamics.  Obviously I couldn't switch back and forth to confirm all of this but even friends who heard both versions also found the TC caps to sound better.  I know this is all anecdotal.  Take it for what it's worth.