5F6-A - Solid State Rectification?

[j-stylez]

I just finished building the P1 from ax84, it worked out very nicely.

My next project is something like a 5F6-A Bassman / JTM45 amp.

I'm recycling some old parts, and with the PT I'm using I wont be able to use a tube rectifier (in addition I also just dont want to use the tube rectifier).

So I want to use solid state rectification, but I'm not 100% sure how to implement it. I'm thinking I could just use 4 1n4007 diodes.
Then using diodes maybe I should use a dpdt standby switch before the diodes instead? Or does it not really matter.
And also will need to watch the voltage with SS rectification, i guess it will be considerably higher.

The next problem is the bias voltage that the 5F6A gets from an additional tap on the high voltage winding.
I need to use one of the regular HV taps. So can i just drop the voltage with a suitable resistor before the diode leading to the 2 220k grid resistors?
I saw something similar on a 1987plexi, that used SS rectification.

I'm thinking I should then also implement a bias pot before the 2 220k grid resistors since my voltages could be off?

I thought it could loke somethig like this:



Would be great to get some of your ideads and thoughts.

[MWJB]

Yes all that would work.

Standby - SPST right after the rectifier will be fine.

I'd use a 50K trim pot in series with a 33-39K resistor (or fine tune the value of the fixed resistor to give say 40-45mA max with your preferred brand of tubes.

The value of your 220K dropping resistor might need some tweaking to perfect bias pot sweep, maybe 150K, 180K etc (@3W)...depends on your final B+. Bigger value = more plate current.

An original 5F6A will show around 470vdc (+/-10v) on the power tube plates at reasonable plate current, with a GZ34. Add 5v or so for SS rectification, so maybe 465-485 might be ball park (345-0-345VAC to 360-0-360VAC B+ winding)? I wouldn't be heartbroken if B+ was another few volts adrift either side of this (a lot of factors affect final B+). RI schem shows 490vdc with SS rectification...in reality they can well exceed 500vdc.

[j-stylez]

Hm...considering the higher voltages, what kind of rating do i need on the filter caps? I still have a lot of 500's lying around.
Whats the usual margin for filter cap ratings? X % of normal operating voltage? Is there something like a rule of thumb?
Really would be nice to use the 500V rated ones. I cant even find any higher rated ones in my usual supply shops.

[MWJB]

Fender also used 500v caps accross the board in the original, but they were anticipating 430odd volts at the plates. As a rule of thumb - 50v rating over the working idle voltage.

Common practice is to rewire the mains like super reverb style, totem pole filter, use 2x100uf (or 220uf) 350/450v caps, each bypassed with a 220K 2W resistor, note that the RI has the screens wired like this too, with 2x47uf...you can also use 2x100uf here. (if you're confident that you will be on the lower end of the B+ range, a single 500v cap will suffice here...if you're not sure, leave a little additional space).

A lot of guys also use another 20uf 500v for the preamp filter on the board, instead of the 8uf 450v.

[tubeswell]

I made one with a 330-0-330 winding and got ~430 with a GZ34 and ~460 with SS diodes. (50uF reservoir). The PT I used was for modern wall voltages

[j-stylez]

Whats my necessary rating on the bias trimpot? I'm having big trouble finding trimpots that are rated greater than 0.75watt.

How about the biasing. In order to be able to bias, I will have to install some kind of bias point.

So considering the fact that I am biasing both tubes at the same voltage, can I put a 1ohm resistor between cathode and ground across which i can check? I would then divide my measurement by 2 to get the current on one individual tube.

Or is my thinking wrong? I looked over Hoffman's layout for the 5f6a, and he puts in an individual 1ohm resistor for each cathode.

If i were to put in 2 individual 1ohm resistors, I would be able to check both tubes seperately, but in the end would still be setting it at the average (i guess?), so that would be the same outcome as using one 1ohm resistor and halving the measured value....

Thoughts please
And thanks for all your help so far! Missing parts coming by mail tomorrow and I'll get started... I drilled up my chassis a minute ago, looking good.

[tubeswell]

If i were to put in 2 individual 1ohm resistors, I would be able to check both tubes seperately, but in the end would still be setting it at the average (i guess?), so that would be the same outcome as using one 1ohm resistor and halving the measured value....

Ideally you want 1 x 1R for each tube, because you can them measure the tube current for each tube separately. Tube current is unlikely to be exactly the same in both tubes for a number of reasons. But on the other hand there is no need to be super accurate. If you what a shared resistor, then put 2 x 1R 1% in parallel as a 'common' resistor for both tubes.

[j-stylez]

One more thing i just noticed. Since I'm recycling transformers from an old amp, my PT is probably not ideal...actually far from ideal. Need some thoughts from you guys again.

I will have to use other rectification than the one stated above, because my HV Tap only gives 200V AC without load. I am thinking that with a diode bridge rectifier I could get around 280V dc roughly... Still very far from the 430 in the original...

My secondaries are 200V (no center tap), 15V (no center tap), 7V (center tap)

So how about using a voltage doubler circuit? The PT is very oversized, so it can handle twice the current wihtout a problem.

Any thoughts on this?

http://www.tpub.com/neets/book7/0301.GIF

http://upload.wikimedia.org/wikipedi...verdoppler.PNG

[MWJB]

0.75W rating for the trim pot is ample, bias circuit draws next to no current.

Voltage doubler would work, some Bogens (CHB?) used this arrangement. That 200VAC might drop a little under load.

[j-stylez]

What do you think of this:



I will add a 10ohm 5watt resistor between caps and bottom tap.

[j-stylez]

And I would rather place the standby switch right after the transformer, because my switch isnt rated for voltages of this height. How about putting the standby switch right at the top tap of the transformer?

One more thing. Since my secondary is around 200volts, I will also have to chose a different value for the 220k bias range resistor. What do you think, something like 140k?

[MWJB]

2x20uf in series will give 10uf total, I'd try 2x80-100uf.

By a decent switch (I'm surprised that it is not capable of handling 400vdc, sure you are just not reading the AC rating?), or use a DPDT on the secondary taps (after the bias feed).

[Wakculloch]

Your bias feed wont work without a centre tapped secondary as it will be sitting at half HT voltage, ie 200v.
You need to couple it through a capacitor to block the DC.
Marshall JCM900's have this sort of arrangement, so crib the values off them.

[j-stylez]

I found the circuit you mentioned. They add a 47n cap followed by 56k resistor to ground before the rectifier diode. Does this eliminate the need for my 200k resistor?

[MWJB]

"You need to couple it through a capacitor to block the DC." Well spotted Wakculloch. Niggez, check out the Bogen CHB schems at Schematic Heaven - FREE Amp Schematics Tube Amp Schematics Valve Amp Schematics Effect Schematics Effect Pedal Schematics Guitar Amp Schematics Bass Amp Schematics Fender Amp Schematics Marshall Amp Schematics Gibson Amp Schematics Vox Amp Schematics under "HiFi & PA Amps" you'll get the idea.

Yes the 47nf cap on the JCM is the coupling cap.