Power Supply Voltage

[Ryan Phelps]

This may not be the best place to float this idea, but restoring Sunn tube amps is where this started for me. So I hope some of you tech guys can assist.
We've all dealt with the issue of modern wall voltage (125 VAC) versus 1960's wall voltage (115 - 117 VAC). This creates higher secondary voltages inside the amp that it was not designed for and places greater strain on the power supply. I've blown current-manufacture can capacitors in several Sunn 200S amps due to the higher wall voltages. This lead me to re-capping Sunn tube amps with discrete caps inside the chassis (totem pole configuration ala Fender for the reservoir caps to withstand the voltage increase). The problem is that you still have higher DC voltages being delivered to every stage of the amp.
One solution is to  increase the size of the dropping resistors in the power supply....this requires large wattage resistors to dissipate the heat, especially at the A node. Other approaches include using a Variac (with the attendant heater supply issue) or using an external bucking transformer, or using a high wattage zener diode between the H.T.secondary center-tap and ground.
I was perusing rectifier tubes today and was reminded of the 5R4GB and its variants. The filament voltage, filament current draw and DC output current closely matches the 5AR4/GZ34 typically used in Sunn amps. The benefit is that the DC voltage drop for the 5R4GB is 63 - 67 volts rather than the 5AR4's 10 volts. This goes a long way to compensate for today's higher wall voltage, bringing the rectified DC voltage closer to the original design specs. The only issue I see is that for a capacitor-input filter (typical in Sunn amps), the maximum reservoir capacitor is 20uf for this rectifier tube. My thought is to use a 20uf/600V or better capacitor after the rectifier, then the 1.5 henry Sunn choke, then 40-uf for the A node, dropping resistor with 20uf/600V capacitor for the B node, another dropping resistor with 20uf/600V for the C node.
Does this make sense, or am I missing something?
I have a 60 watt Sunn amp on the bench now and I think I will try this.

Thanks guys!

[BassHog]

FYI there are higher voltage cap cans available now from a couple of different places that have a 600V rating. Dynakit parts has one.
I personally have never had a problem with the 525V CE Man cap cans in 60 watt Sunn tube heads as the voltages in them run a bit lower than the 40 watt heads.
I usually use the 600V can for recapping the 40 watt heads, and I give the customer the option of either the 525V can or the 600V can for the 60 watt heads. I personally have a Sceptre with a new 525V can in it that also runs a SS rectifier and it has been running strong for a few years now with no problems.

I don't find any issues in the amps at all related to the higher DC voltages as far as performance goes, so I don't see it as a problem and have never had a reason to try to lower the DC voltages. For amps used for bass I actually like to use an SS rectifier instead of the tube to give a bit of extra voltage and get a bit more power out of them.

I use a different method for recapping the big heads like the 2000s or 350b, but that is another subject.

[Ryan Phelps]

Thanks BassHog!  I have not found that can cap in the past. I see that it is 550VDC working voltage and 600VDC surge which should work in most cases. My wall voltage in the Seattle area can be 126VAC or better and I've lost two 525VDC caps in different Sunn amps. The first stage of the can cap can see 560 VDC (instead of 480 VDC per the schematic), so they are working on the edge. YMMV depending upon your local wall voltage.
I've had problems with excessive voltages at the 6AN8...actually fried a couple. Hence my search for ways to bring power supply voltages closer to original spec.
I did try a 5R4 rectifier tube and it did pull the B and C nodes down closer to original spec. I suspect tweaking the dropping resistors slightly higher would help too.

[Soundmasterg]

There are quite a few different versions of the vintage Sunn amps, and each has different voltages. The earlier amps with Dynaco transformers have lower voltages than the later amps with the Schumacher transformers for instance. The amps intended for guitar rather than bass tend to have lower voltages also.

When I recommended the series connected caps with parallel resistors method in the past, that was when the only other option was the CE 525v can caps, which are not sufficient. It is still a good method in that it will always work, and it makes future replacement easy and cheap. If there are better rated can caps, that works too, although can caps are more expensive and create grounding compromises, though if the amp is not designed to use discrete caps and galactic or star grounding, as most vintage amps are not, then using a can cap doesn't create any further problems.

I modified a later 200S that had the Schumacher transformers by using different value dropping resistors to get the preamp voltages closer to the Dynaco models. I found that keeping the node that supplies the phase inverter as the stock value resulted in more power and a better sound for bass. Changing the ones following that supplied the preamp to higher values, thus lower voltages allowed for a softer and less hard sound out of the amp that I liked. That amp has since moved on but it is also a good approach.

Greg

[Ryan Phelps]

Thanks Greg! Your kind reply is appreciated! I love the sound of these old amps and sharing our restoration knowledge helps us all.