5F6-A - Solid State Rectification?

[niggez]

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.

[niggez]

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

[niggez]

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.

[niggez]

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.

[niggez]

What do you think of this:



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

[niggez]

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.

[niggez]

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.

[niggez]

Ok that was a very nice idea. The bogens do exactly what I want!
So I'm adding the coupling cap. Whats the purpose of the 56k resistor to ground after the coupling cap? Is that an alternative to using a series resistor of something like 200k?

[MWJB]

Yes, really. You're now looking to drop rather less voltage from the AC source (from 350 odd down to 200), so I wouldn't hold on too much to the idea of that 200K dropper. I'd probably copy the CHB 50 Bogen circuit and sub the grounded 68K after the bias diode with a 100K trim pot, wired as a variable resistor. Find the value that allows you to bias to say 45-50mA per tube MAX, then insert this value between the trim pot & ground. Or use 68K from the Bogen schem in series with a 50K trim pot (wired as above).

Set your negative voltage at 6L6 pin 5 BEFORE installing the power tubes, maybe -50vdc at 400vdc plate voltage, -60 at 450vdc plate voltage...this should make sure that you are not under biasing the tubes at initial installation. Or, if bringing up on a variac, with tubes installed - start at 1/3 wall AC (you will then see 1/3 plate voltage at pin 3 & 1/3 neg dc voltage at pin 5) and monitor as you bring up the volts.

[niggez]

Actually, I'm not dropping down from 300 something, but from 200 something.. because the bias circuit draws off of my tap running on 200 (190 or something like that with a load...)
So right now I put a 56k resistor with a 25k trimpot to adjust the bias sweep, and a 50k bias pot with 33k to adjust bias. I'll see how this works out once i have everything hooked up, before installing any tubes.
I'll finetune it, maybe sub the 58k or the 33k, I will see what I have to change.
Hope its going to work out. Thanks for all the help again.

[niggez]

Allright... I got it all wired up.

Problem now: I'm getting about 540V (no tubes installed) after my voltage doubler rectifier. Thats too much.

Any ideas what I could do to lower that a bit to get closer to 430?

[MWJB]

"Problem now: I'm getting about 540V (no tubes installed) after my voltage doubler rectifier. Thats too much." How do you know if you have no tubes installed? Try and get -64vdc to -70vdc at pin 5 of the 6L6s, install your tubes and bias to 30mA per tube. Then tell us what voltage you have at the plates.

I have seen amps that run 490vdc @ 30mA on the plates rise to 540v at lower currents, even with all tubes installed.

"Any ideas what I could do to lower that a bit to get closer to 430?" Why would you want 430vdc if you are building 5F6A style amp?

[niggez]

I thought my goal should be 430 with tubes because the original schematic states 432 Volts at the first filtercaps.
I didnt know this much more was ok, but it is nice to know this.

Anyway, I'm still trying to get the bias circuit working.

It doesnt work though, I'm getting 0 V from it. I looked over the Bogen and Marshall schematics again, but cant seem to find the problem. I thought that my voltage wouldnt be ideal yet, at least I was expecting a voltage I could work with.

Before I troubleshoot more, can someone spot an error here?

And does the cap before the diode have to be one with polarity? I'm using a film cap.

Its wired like this:

[MWJB]

Do you have 200VAC before the .47 cap (non polarised is fine)? Do you have VAC at the bias diode and vdc after it...at what point between the B+ secondary and your 220K grid resistors does the bias voltage disappear? Are you sure that you are simply not getting an overload reading, your voltage divider does not seem to drop enough voltage to work with your power tubes (it would seem that you are still hitting the grids of the 6L6s WITH -250vdc, rather than -60 or -70)?.

Why not copy the Bogen schem, with the trim pot as I described?

[Steve Conner]

I'm not surprised that won't work, there's no DC path for the diode current.
There, fixed.
Edit: Oh, but it still won't generate any bias when you're in standby, so there'll be a big surge of plate current when you flip the standby switch. A NOS VINTAGE RARE cookie for anyone who can figure out how to fix that.

[pdf64]

Not a perfect fix, but if current in-rush suppressors (NTC thermistors) were fitted in series with the ground returns of the power tube cathodes, then at least some cathode bias / current limiting would occur at startup.
It might be difficult finding parts with just the right spec though.
The old 'brimistors' in AC50s would be perfect.
Perhaps have a 3 position standby switch, arranged off/cathode/fixed, which switched in some suitable cathode resistors in the middle position, then shorted them in the 'fixed' position.

[Steve Conner]

I think a better plan would be to rearrange the circuit so that it still generates bias when the standby switch is open. *ahem* single pole standby switch

[pdf64]

Damn - that cookie is just playing hard to get!

[niggez]

Which tap should I put the SPST switch on, the top part in my drawing? So I simply remove the bottom part of my dpdt switch in my drawing?
Because I thought that would still leave me in the same situation.

So I guess I could use SPST switch after the voltage doubler, but then I would first have to find a 600v dc rated switch...

And MWJB, I actually didnt look at the right bogen schematic. The 50 was so fuzzy that I kept looking at the others.

[Steve Conner]

Yes, just delete the bottom pole of your standby switch. Then the bottom end of the PT winding goes back to the centre tap of the two series capacitors, and that completes the circuit for your bias tap. The way you had it, with the standby switch open, the bottom end of the winding was just dangling in the breeze.

Re the 600V DC rating: If you get a double pole switch and connect the two sections in series, that doubles the voltage rating. Twice the contact gap means twice the arc breaking capability. So I suggest you do that even if you're breaking AC.

[niggez]

Time to celebrate, the bias circuit and voltage doubler now work. The whole power supply section seems to be working fine. Thanks again for all the help so far (and more to come )

1.) Bias

Plate voltage of the 5881's is still +535V, my tubes are Sovtek 5881WXT.
Now to the biasing: went through first power up, and took some voltage readings...

Voltage across 1ohm bias resistors are:

3mV at -75V bias

15mV at -65V bias

1 ohm resistors are 5% wirewound, the voltage across them differs slightly. I think its still within margins, for the second measurement (15mV) it was 0.6mV difference between the tubes.

I couldnt go lower than -65 with my bias combination, so I added another 68k in parallel to the 56k before the bias trimpot, because it was a bit hard to reach the 65k and replace it by a lower value. I will replace the 68k with a higher value to give me correct sweep once I've found the voltage that gives me the maximum plate current I want to be able to dial in.

22mV and 24mV at -60V bias

The difference has increased, but I believe its mostly due to my 2 not-so-1ohm resistors.

At 535 plate voltage I would be at 0.023A*535V = 12.3 Watt
Maximum dissipation is 23 Watt for the 5881, so I was planning to go up to 15W/535V = 0.028A

2.) Noise / Shielding

I definately have to get some shielded wire in the preamp section. I dont have much left, so I will replace the signal path to the first preamp tube by shielded wire first. There is quite a bit of noise.
Also, I read its a good idea to lift the 68k resistors from the turretboard and attach them directly to the input jacks. Would you recommend this?
Or, could I run only one wire for each of the two channels to the turretboard and make the connection of the 68k's there for each channel to further reduce noise? I dont plan to use inputs 2 of both channels at all.

Im running 12ax7 in v1, because my 12at7 / 12ay7 havent arrived yet. (out of stock :/ )

3.) Filament 6.3V heater center tap

Does the filament center tap need to be grounded? I wasnt sure about this. I am thinking that I might be getting hum because its not grounded?

[MWJB]

1) Bias - as you increase the plate current (yes 28-30mA per tube sounds fine) your plate voltage should drop. The nominal difference in plate current you have is not a concern.

2) 68k on the input jacks woks fine in millions of amps.

3) Either ground the filament centre tap, OR fit a 100ohm resistor from each leg of the filament secondary to ground (virtual centre tap). Usually the most convenient place to implement this is at the pilot light.

[Wakculloch]

Just come across this thread again, and remembered that the man from Marshall who i spoke to about the bias coupling cap recommended using one that was designed to fail with a short circuit (class Y ?), so that if it did stuff up, bias would increase rather than disappearing.
I don't know if they said this as a precautionary measure or if they'd had problems, so i can't put any weight on his comments, but he was pretty insistant.

[niggez]

Allright, I'll see if I can find a cap of that type.

Biased to 28mA, plate voltage is at 490V.

The amp sounds great. I replaced the 2 inputs with shielded wire, as well as the pin 2 cable on the second preamp tube. It is now completely quiet, almost dead silent.

Controls all work fine, only one problem I still need to fix: At high volumes I get crackling/popping noises. There is none of that at low to medium volume, as soon as I turn the volume up past a certain point, the popping occurs when I hit a powerful chord.
Then turn up the voume some further, and the popping/crackling starts even when I hit the strings only a little harder.
Any ideas how I can try to trace the source of this problem?

[MWJB]

"Controls all work fine, only one problem I still need to fix: At high volumes I get crackling/popping noises. There is none of that at low to medium volume, as soon as I turn the volume up past a certain point, the popping occurs when I hit a powerful chord.
Then turn up the voume some further, and the popping/crackling starts even when I hit the strings only a little harder.
Any ideas how I can try to trace the source of this problem?" Shorten all grid wires, shield them if necessary (even to the power tubes). What grounding scheme have you used? Any pics so we can see your layout?

[niggez]

Here is what my grounding looks like at the moment:

Pots are grounded to the chassis, same is true for the speaker jack.

Black dots means connection to chassis.



I first had the turretboard grounded to the point on the right side, but then changed it to the drawn diagram (its connected to the chassis close to the input jack now).

I started to read a document on grounding on the valvewizard site, and came to the conclusion that my grounding isnt ideal yet

I tried to copy the grounding of the jtm-45, but I'm not sure where to connect all the star grounds. If i run them all to one point, my wires would be quite long and going all over the place is what i thought.

Should I ground the turretboard on the right side, to the star ground? And if so, should I leave the connectin of turretboard to input jack ground? I thought this would make a groundloop.

But when I still had the turretboard grounded at the right side, it was much quieter.

Also, is it a good idea to run a cable from the speaker jack to the star ground on the right side? Its currently grounded to the chassis only.

[MWJB]

Your grounds from the board should not be daisy chained, run each wire separately to the input jack ground.

Preamp and PI filter caps ground to the input jack ground. Screen & main filters to main power amp ground.

Why not ground the OT at the speaker jack? It works well enough on ALL 5F6As.

Do you have a buss wire for the control pots?

I can't tell from this how long things like your grid wires are, or how well components on the board line up with threir respective tube pins etc.