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I've read this thread with interest as I have been working on a ST35 monoblock pcb using the 5Ha7/4ha7 and configurable for el84/6gk6s. I forced myself to learn DesignSpark PCB (free with no limitations) and should have the board ready to be fabbed next week.

This was more or less an experiment to learn how to use the pcb program and have a few boards for myself but there will probably be extras available. The board will be 10cm x 10cm.

That sounds cool although I prefer to wire mine point to point and have the sockets mounted to the chassis.

Did anything ever become of this? I am thinking of revisiting the monoblock theme.

I never had the boards made but all this ST35 talk has gotten me thinking about it again. I'm not sure if I should go two separate boards (monoblocks) or one bigger board (but not as big as two mono boards would be).

I also designed the boards to use variable fixed bias instead of variable cathode bias.

Sadly, I've put this idea on hold for now and have been working on other projects. Still want to do it.

So I started the first one.

http://i576.photobucket.com/albums/ss20 ... 0bgnv0.jpg

I'm still waiting on a few parts.
http://i576.photobucket.com/albums/ss20 ... h3qcqs.jpg

http://i576.photobucket.com/albums/ss20 ... tzrhyp.jpg

Hopefully I can get this one working so I can start on the next one.

It's alive! Now I just have to build another one.

http://i576.photobucket.com/albums/ss20 ... sk0iuw.jpg

http://i576.photobucket.com/albums/ss20 ... yaqk3p.jpg

How does it sound?

For their value, those are the LARGEST film caps I have seen!

It sounds great! I splurged on some parts: Vampire copper RCA jack, Vishay Z-foil input resistor, Mueller copper binding posts, and brand new Clarity Cap CSA series caps. I've been looking forward to trying the new Clarity caps and the CSA is definitely improved over the previous ESA series. I found the latter to sound great in the bass and mids but seemed muted in the treble region. The CSAs are much better balanced and a great bargain.

I only have JJ driver tubes and Valve Art EL84's so there's room for improvement there. I was a little concerned about my layout in such a small box (7x7x2) but it seems to work ok. With the input grounded its dead quiet. I should also mention I'm using Dave Gillespie's EFB circuit. I'll report more after I get the second amp built.

Picked up a matched set of Tung-Sol EL84 tubes from Jim McShane. Big improvement over the Valve Art tubes I used for testing. These little amps pack a sweet punch!
http://i576.photobucket.com/albums/ss20 ... o2rxpt.jpg

Do you have basically the same plate voltages on the tubes as in the original ST-35? Did you double the resistor values in the power supply chain since the current draw is now halved? I'm curious about this. They look pretty cool!

Stunch wrote:Do you have basically the same plate voltages on the tubes as in the original ST-35? Did you double the resistor values in the power supply chain since the current draw is now halved? I'm curious about this. They look pretty cool!

To be honest I hadn't accounted for that. I simply built an ST35 minus a channel in each box. I thought the voltage was a little high even considering I have them running at 117VAC through a variac. I had no trouble setting the idle current to .27mV. The PT feels cooler at around 99 degrees. They sound great although they're still breaking in.

Here they are in action.

https://youtu.be/bJa_9mDHcWc

This looks like something that would interest me. I'm curious as to what B+ readings are in your configuration.

Since the current draw is cut in half, I guess you would have to double resistor values in the supply chain to mimic original values, and maybe use a power transformer with less voltage on the secondaries. Or maybe use a rectifier tube to knock down the voltage.

If you have a moment, could you take a voltage reading on the EL84 plates? I'm curious to know what it is.

Thanks

Stunch wrote:This looks like something that would interest me. I'm curious as to what B+ readings are in your configuration.

Since the current draw is cut in half, I guess you would have to double resistor values in the supply chain to mimic original values, and maybe use a power transformer with less voltage on the secondaries. Or maybe use a rectifier tube to knock down the voltage.

If you have a moment, could you take a voltage reading on the EL84 plates? I'm curious to know what it is.

Thanks

AC: 116
EL84 Plate 386 (For a stereo amp this should be 360-370)

AC:115
EL84 Plate 384

Clearly I need to change something to get the numbers back to where they should be. I'm still working on understanding Ohm's Law but it's not as easy as it looks. I've never had any formal electronics training. All self-taught.