A SSS Build

[martin manning]

Completed installation of bipolar power supply. Amp is now working again with better B+ and other voltages.

Question: with the decreased voltage the bipolar supply provides V7 does this limit the clean headroom? Volume? Is there a noticeable tonal difference between the original and bipolar supplies?

I wouldn’t think so, and reports from others seem to agree. I assume you are aware of the associated resistor value changes in the bias circuit and driver cathodes, and see this post about the eyelet board version in the files section where a small resistance is added to the AC input, and a bleeder resistor is added to the positive side: https://ampgarage.com/forum/viewtopic.p ... 23#p463923

[timrobertson100]

Congrats Glenn - they are fabulous amps, right?

In the coming months, I'll get to building a second one, using the parts bundle I bought from a TAG member here. Can you please post pics of your bipolar supply setup?

[GAStan]

Completed installation of bipolar power supply. Amp is now working again with better B+ and other voltages.

Question: with the decreased voltage the bipolar supply provides V7 does this limit the clean headroom? Volume? Is there a noticeable tonal difference between the original and bipolar supplies?

I wouldn’t think so, and reports from others seem to agree. I assume you are aware of the associated resistor value changes in the bias circuit and driver cathodes, and see this post about the eyelet board version in the files section where a small resistance is added to the AC input, and a bleeder resistor is added to the positive side: https://ampgarage.com/forum/viewtopic.p ... 23#p463923

I did see that post after I had the bipolar installed. The 33R was added to the bias tap but not the 330k bleeder because I haven't found a suitable place to install it. I may put one on the PI board but I am very wary of the charged caps in the meantime. The divider resistors were also changed. The resistor in series with the pot is 10k because the pot is actually 25k, originally a 10k that I transplanted a 25k element into. Lacking 75k I used a 68k (measures 69k7).

Congrats Glenn - they are fabulous amps, right?

In the coming months, I'll get to building a second one, using the parts bundle I bought from a TAG member here. Can you please post pics of your bipolar supply setup?

Note the 33R resistor in black heat shrink on the bias tap input to the board.

If using Erwin's chassis be aware of the close proximity of the PT and end wall of the chassis, makes for a tight fit depending on where you plan to mount the board.

It's a pretty straightforward install but pay attention to Martin's comments above.

Let me know if you need more detailed pics and I'm happy to get them for you. The cabinet for this won't be delivered for another month or two so it's no trouble to click and post pics in the meantime.

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[GAStan]

I've been working on an oscillation. It's only in the power section, from the PI to speakers. Goes away if feedback from speaker is disconnected.

First three of the below are
Channel 1: 8 ohm dummy load;
Channel 2: PI (V6) Pin 6.
400 hz signal.
8 ohm dummy load.
Refer to Viperdoc's schematic.

Pic 1
This is how I originally had it phased, wired according to wire color of OT. Note the voltage levels, 30 volts into 8 ohms. Extreme clipping, looks like a squarewave. 44hz. Amp was buzzing, I believe this is what caused the possible damage to the original PT. I only had the feedback connected long enough to save this on my o'scope.

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Pic 2
Same phasing as above but Feedback from 4 ohm speaker tap disconnected. Signals out of phase. If I understand correctly this should be correct phasing for negative feedback.

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Pic 3
Here I have reversed wires from CF (V7) to power tubes. No feedback. Signals in phase.

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Pic 4
Same phasing as Pic 3. No signal in, volumes on zero. Channel 1 is 60hz noise, I am literally simply touching the probe tip to my finger just to get a 60hz trigger. Channel 2 is PI pin 7. WITH Feedback. Higher frequency varies between 100Khz and 150Khz. I added 220k gridstoppers to CF, it only decreased the frequency from 150Khz range to 100Khz range.

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I'm unsure which phasing is really correct, picture 2 or 3 above?

[GAStan]

Found a problem. Hopefully the problem. Circled 270k should be 270r. Voltages on V6 are much higher than on Viperdoc's schematic. Will work on this more tomorrow.

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[martin manning]

Hope that solves it!

The speaker output should be in phase with the PI input.

Yours will be the first with the new resistors added to the bipolar supply. I’ll be interested to hear how that works in hardware.

[GAStan]

Hope that solves it!

The speaker output should be in phase with the PI input.

Yours will be the first with the new resistors added to the bipolar supply. I’ll be interested to hear how that works in hardware.

It did fix the problem, thanks.

The bleeder is definitely a good idea. The +170 volt caps were still at 50vdc after sitting overnight before I added it. Next time I power it up I'll get voltages and post them.

[martin manning]

The bleeder is definitely a good idea. The +170 volt caps were still at 50vdc after sitting overnight before I added it.

Credit Tony D for that idea.

[martin manning]

The bleeder is definitely a good idea.

I made a small tweak to the eyelet board .pdf on this. Increased the current limiter to 47R, and decreased the bleeder to 150k. Should have it drained in a minute or so.

[GAStan]

The bleeder is definitely a good idea.

I made a small tweak to the eyelet board .pdf on this. Increased the current limiter to 47R, and decreased the bleeder to 150k. Should have it drained in a minute or so.

Thanks for the heads up, I'll check it out.

[GAStan]

I'm having a cabinet built for this project and have a question. How much space should there be between the bottom of the PT and top of the Reverb tank?

[martin manning]

I would start with the proportions from the real thing to get the height of the box, and do an experiment by stacking the chassis and tank up on the bench to see if there is any problem. Keep the output side of the tank close to the input side of the box since it is most sensitive to picking up hum.
http://www.jimsguitars.com/s/3888/Ampli ... Singer.htm

[sluckey]

I'm having a cabinet built for this project and have a question. How much space should there be between the bottom of the PT and top of the Reverb tank?

They can be closer than you would think. Look at this Sunn...

[rccolgan]

I'm having a cabinet built for this project and have a question. How much space should there be between the bottom of the PT and top of the Reverb tank?

I'm fairly certain the real deal is 11.5" wood-to-wood height as a starting point for ya. That's what I used with mine and the proportions look good and no reverb hum or oscillations.

[GAStan]

FINALLY received my new cabinets for this amp. Delay was combination of things including changes I made to original plan, wood sourcing issues and various other things. Very well packed, they arrived undamaged which is saying a lot considering my experiences with our local UPS service. However I'm very happy with the end result.

Cabinet carcasses are Sapele. Kurt Wyberanec, the builder, sourced some beautiful wood. Photos do not do it justice. The grain catches the light and gives it a near 3d effect as you move. Joinery is tight making these very solid. Each case is from a single board so grain pattern is consistent from side to top to other side. Maple beading and port trim. Cocobolo faceplate on amp cabinet polished to high gloss. Sapele has satin finish.

Big THANK YOU to those of you that replied to my last question about height and Reverb tank placement. After bench testing I told Kurt 8 1/2" minimum wood to wood with max of his choice to get a pleasing dimension. I haven't measured but it looks like ~9". Reverb fits nicely without hum.

I sent Kurt a bunch of photos and measurements in lieu of the amp itself. Head cabinet came with holes drilled. The completed chassis bolted right up without issue.

Speaker sounds great. It came loaded with some 12" 75 watt 8 ohm heavy magnet Creambacks. 66 lbs according to UPS. I have a pair of EVM12L'S I intend to put in it in the near future. The Creambacks will go into a 2x12 oval back solid pine cabinet I am having built to Dumble specs found elsewhere on this site.

This amp is kind of an early retirement present to myself, it will adorn our living room for years to come ready to be plugged in and played at a moments notice.

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