PT Thoughts and Assistance

[defaced]

So I roasted a perfectly good PT last night. I think I wired the 20-0-20 secondary wrong and up in smoke it went. So, now my room smells like burnt transformer and I'm in the PT market. The problem I'm having is that I'm not finding anything out there with the specs I want, so I want to see how far I can smudge the rules with what I've found.

Here's what the dead PT had:
360-0
6.3 @ ~3A
60-0
20-0-20

The major problem I had with this PT is that there wasn't enough heater current for 4 x KT-88 and several preamp tubes. I found some toroidal PTs on eBay from AnTek which look nice for the HT and the heater, but no bias tap and no lower voltage to power switching stuff.

Here's what I'm looking at:
http://www.antekinc.com/AN-4T360.pdf
and
http://www.antekinc.com/AN-4T400.pdf

Each have two HT windings at 500ma each and two 6.3v windings at 5A each. Each of these are 400VA transformers.

So here are my questions:
-Is there any easy way to get a bias voltage from this tranny? I've thought of two options:
1) Add a 60v secondary winding for the bias.
2) Use a stacked power supply to get a negative voltage

-Is there any problem with mounting two toroid concentrically with one on the outside of the chassis and one on the inside? What I'm thinking is I could buy one of their 25VA, 2 x 12v trannys for my channel switching stuff and mount it on the same bolt as the main toroid but have it on the inside of the chassis.

-If it's possible to mount two transformers on the same bolt, would it be a good idea to buy a 25VA, 2 x 24v tranny and use one secondary for channel switching and the other secondary with a voltage tripler for bias? Or will this create a DC offset in the transformer because the current from each winding isn't identical?

-Is there any advantage/disadvantage with using the 400v tranny with the secondaries in parallel which will give me ~560v after my bridge rectifier? The OTs I typically see for p-p 100w-120w amps have a primary impedance of about 2k ohms, but I don't know if 4 x KT-88s at 560v in class AB with a 2k impedance would be a good operating point. I'd also like to play with ultra linear operation at some point once I get a different OT (we'll cross that bridge later), but I don't know enough about designing output stages to know if I'm making an obvious mistake or not.

[black_labb]

you could get the bias from the HV tap as some amps do. its usually done by putting a resistor followed by a reversed diode then some more resistors and a voltage devider (and a cap or 2 to smooth ripple). not sure about using 2 transformers so close together.


and using a PT with only 3A of 6.3v for 4 kt88's is way underrated. they use 1.65A each. the antek's look to be a good idea for 4 kt88s.


edit: actually you should be able to use the 2 winding's in parallel, as they are not linked. you will need to make sure that you get them in phase, otherwise you will blow another PT. measure the ac voltage difference between the 2 windings when 1 end o each are connected. if the difference is 0v, then you are ok. i'd put a small quickblow fuse in just in case.

[defaced]

Yea, on the bench I was using 2 x KT-88 and a second heater tranny to keep things reasonable.

Thank you for the idea about taking the bias from the HT. I had thought about that but wasn't sure how to do it. I think after reading your description and thinking about it I've got a good picture in my head.

[defaced]

Well, the picture in my head doesn't work, and the more I think about it after seeing the bias pulled from the HT in a schematic, the more I think that it'll induce a DC bias into the toroid and hum. So... I'm still a bit lost on this solution. But I think I found another...

I found a Good Will special for my relay/switching stuff: an HP 32v, 1.5A switching power supply Now, back to bias, inside this thing there are two transformers, of which one of them is a mains isolation transformer. I think it's secondary might be a good place to pull my bias voltage from. Thoughts?

As for the increased HT, I'm still not sure. I've plotted the load lines and my inexperienced eyes don't see a whole lot of difference. Basically it looks like for the same operating power I'll need a little more bias voltage, and I'll use a little less HT current. I still wouldn't mind some advice on this one, but I think I'm just going to buy the 400v transformer and see what shakes out. I'm actually more concerned with the standby switch than anything else.

[andrew_k]

I found a Good Will special for my relay/switching stuff: an HP 32v, 1.5A switching power supply

You could get your bias by running a voltage doubler from the 32V transformer. I'm doing a similar thing with a 24V xformer and a voltage tripler for a pair of KT88 - http://notinteractive.com/stuff/guit...er/psu-1.1.gif
Works fine, and can supply up to -100V if I want it. Bias range is enough for 25W tubes (KT77) or the 42W KT88.

[defaced]

I thought about that too, and was hoping it would work, but no dice. The two trannys inside this thing are no where near 32v. One is the mains isolation transformer, the other bridges between two parts of the switching supply and give 500v AC on one side, and I don't get a measurement on the other. So the output is a nice clean 32v DC, and from my brief look online, there are chips to increase voltage, but they require more parts and planning than taking the bias from the isolation transformer.

More thoughts. Thinking of Black Labb's suggestion of pulling the bias from the HT, couldn't I use a coupling cap to keep the DC out of the transformer? If my math is right, and I figure the PT secondary is 10 ohms, then I'll need at least 265uf of capacitance to pass 60Hz. Now, does such a beast exist? I'm thinking a typical aluminum electrolytic wouldn't work because it's polarized, yes?

[Steve Conner]

As far as I know, the bias current is negligible, something like 5mA, and shouldn't cause your PT to saturate.

You can put a capacitor in your bias dropper if you really want, but you'll have to think carefully, or there will be no path for the DC and your bias supply won't work at all. If you rectify it with a single diode (and you will) then you need to connect another diode to ground in the opposite direction to complete the DC circuit.

265uF is a little large, I'd guess nearer 0.22 or 1uF.

[black_labb]

then again, is there any issue with using a FW or bridge rectifier for the bias? its not needed, but 1n4007's are cheap, and just to be sure. just put the resistors in before both diodes.

here's an amp i built. theres a schematic about half way down. in the power supply section you can see the bias supply. http://music-electronics-forum.com/s...ad.php?t=10743

is there any issue with using half wave rectifiers with bridge rectifiers? do the windings become referenced to specific potentials through the diodes making a hw and bridge not work correctly?

[Steve Conner]

Thanks for linking to that schematic Black labb.

I'm talking about replacing the 160k dropper resistor in your bias supply with a cap, or connecting a cap in series with it, as this is what's needed to stop DC flowing in the transformer. It will also allow making the resistor smaller to get more current.

But if you look at the schematic, you'll see that the capacitor also blocks the DC load current, so the bias voltage would collapse to zero.

Placing another diode from the junction of the 160k and bias rectifier cathode, to ground, should solve this. (Cathode to ground.)

By the way, I have a big pile of ECF80 and ECF82 tubes. I just realised one of those is the same as the 6BL8, so I'll need to try a circuit like yours.

[defaced]

Excellent. I will try to pull the bias from the HT with and without the cap and report back.

Do either of you have any thoughts about increasing the HT voltage from 500v DC to 560v? I'm on the cusp of pulling the trigger but wouldn't mind some discussion this because I'm a bit fuzzy on the practical changes that will result.

[black_labb]

increasing the voltage would be a good way to get some more power. the impedance is fine, you may end up getting towards 170W from the thing, so dont skimp on the OT. hammond OT's seem pretty rugged and are quite under rated for MI use. a 1650T (think its the one, 120w 1900 ohms impedance) would probably be a good bet. the transformers would be plenty for 4 kt88's.


steve, do you have any ideas for how i can a bias voltage from this transformer?


~~~~~~~~~~~~~~
|______|___________|
0v____40v_________300v

i am running a bridge rectifier between the 0 and 300v, and planned on using the 40v for bias, but i dont think it will work after looking more closely. i suddenly got the urge to rebuild my first amp (5e3ish) as it was built into a chassis too small and badly organised. i decided i want to put a fixed/cathode bias switch, and would like to use a bias supply as opposed to a zener.
Could using a cap from the 0 and 40 then ground the 0v an use a HW rectifier work? i keep thinking that the caps will mess around with the referencing, but then i think otherwise.

give the ecf80/6bl8's a go, i'm curious how you like them. i kind of wish i put some 12ax7's or similar into another channel just to see how they differed, but i didnt. i am happy with how it sounds either way.

[Steve Conner]

Hi black labb

I'm not sure how the bias tap could be made to work with a bridge rectifier, unfortunately. The reason is that the bridge rectifier alternately connects one or other of the ends of the winding to ground. So your 40V tap will be a 260V tap half a cycle later. The bias dropper described in this thread won't work either, as the bridge rectifier never allows either end of the winding to go negative.

One possibility might be to connect the bridge rectifier between the 40V and the 300V, instead of the whole length of the winding. Then the 0 tap will go negative on alternate half-cycles and you can connect your bias rectifier diode to it. But this reduces your main B+ voltage (the bridge only gets 260V AC instead of 300) and you might not want that. Then again, in a 5E3 style amp, it might actually improve it.


About the voltage question, yes increasing the voltage on KT88s should provide a bit more power with no ill effects. Even new production tubes should take 560V, NOS ones would still be loafing around in their pyjamas.

I also have some ECC88s and an OPT from a Sound City 120, and can't decide what to do with those either.

[black_labb]

actually i got it slightly wrong with my diagram. the PT i am using is this https://taweber.powweb.com/store/PTGPsch.jpg . it has some different possible combinations. the 40v is actually on top of the actual voltage, so its 0v-40v-340v (between the 410, 370,70 taps respectively) where i was going to use it. so i should be able to use the idea you mentioned. just then
what i was planning on doing was using the 300VAC into a bridge rectifier. when the amp was in cathode bias mode there was a resistor (47ohms was what i was using) before the plate voltage to give it some sag and get the voltage at what a 5e3 would usually be. when i switch to using fixed bias it bypasses the resistor for a stiffer sound and more power from the higher voltage. after looking at the switching, i need to put the voltage changing resistor onto a different switch (unless i can get a large 3pdt switch to fit in the hole i made). i'll just put it onto the standby switch and make the standby a on-off-on switch, this way i can get the high voltage in either cathode or fixed bias.

ps, i modeled the bias supply in lt spice, and it works, thanks for the tip.