Hiwatt 200 bias problems...

[Dingleberry]

My friend gave me his hiwatt to rebias because it's biased way too hot.
It now has 4 x winged c 6550c:s in it.
I decided to do the Mark Huss bias doubler circuit, but now I read the bias voltage betveen 69V - 97V.
Way too much and the power tube current is something like 2mA.
Yeah I know that seems very low and believe me or not I had to check my bias probes with another amp to see if they are working correctly.
They are and now with the bias pots set to the hottest end of the travel I read only that little current and the plate voltage is now nearly 740 volts so you can tell from that too that there is no or just a little current flowing.

So now I think I need to lower the voltage...

What is the "best" and SAFEST way to lower the voltage.

That model is equipped with own bias winding so I could probably wire a full wawe bridge there to get the voltage little lower than with the doubler or am I right?

Would adding some series resistance before the bias pots give me a different range? Correct me if Im wrong but resistance before pots and the bias pots in conjunction with the "tail" resistors form a voltage divider.

Now when I add the resistance before the pots I shift the dividing point towards ground. Something like 10-22k would probably be good value?
There is picture attached.

I'm using 10k pots wihout those 220k resistors and uf4007 diodes instead 1007 diodes.

-T

[Hellhammer]

Yes you're on the right track. I'd switch to 25k pots first, for bigger sweep, and see where that leaves you as you'll be able to bias them hotter if you do. And in case your friend wants to try KT88's next you might need more negative voltage. Then add resistance where your arrow is pointing. 10k is probably a good place to start. And do add the 220k's for insurance in case of pot failure.

[FUCHSAUDIO]

Yes you're on the right track. I'd switch to 25k pots first, for bigger sweep, and see where that leaves you as you'll be able to bias them hotter if you do. And in case your friend wants to try KT88's next you might need more negative voltage. Then add resistance where your arrow is pointing. 10k is probably a good place to start. And do add the 220k's for insurance in case of pot failure.

The potentiometer value is a little low, but not the source of the problem. You have no resistance on the top of the tops, but you have a tail resistor. This means the pots are on the high end of the available voltage. Your picture shows about 25-K total load (25-K pot plus tail times two in parallel), so a resistor of equal value at the point you marked in your diagram would make the bias supply about half the supply. You could put a 50-K potentiometer series with the pots, put your bias pots in the middle, and adjust the added pot so your bias is in the mid range of what you are looking for. Remove the pot and measure, then install the nearest standard fixed resistor value.

[Hellhammer]

Of course it's not the source of the problem, the source of the problem is too high neg voltage.

Yes you could just add resistance in series with the pot.

Anywho as I said that's how I would do it. 25k pots should get him to a range of about -50V to -100V. Add a little resistance in series and you can bias both KT88's and 6550's.

Isn't the screen voltage a bit high for 6550's by the way? Over 400V IIRC...

[Dingleberry]

Yes you're on the right track. I'd switch to 25k pots first, for bigger sweep, and see where that leaves you as you'll be able to bias them hotter if you do. And in case your friend wants to try KT88's next you might need more negative voltage. Then add resistance where your arrow is pointing. 10k is probably a good place to start. And do add the 220k's for insurance in case of pot failure.

The potentiometer value is a little low, but not the source of the problem. You have no resistance on the top of the tops, but you have a tail resistor. This means the pots are on the high end of the available voltage. Your picture shows about 25-K total load (25-K pot plus tail times two in parallel), so a resistor of equal value at the point you marked in your diagram would make the bias supply about half the supply. You could put a 50-K potentiometer series with the pots, put your bias pots in the middle, and adjust the added pot so your bias is in the mid range of what you are looking for. Remove the pot and measure, then install the nearest standard fixed resistor value.

Thank you for fast replies Hellhammer and Fuchsaudio!

Yeah I know the pot value is kind of low (10k).
I was thinking about that "pot trick" too;) It's always good that someone confirms your suspicions. Took also one basic electronics book out of the shelf and carefully read the chapter "voltage dividers". Now and then it's good get back to basics and sometimes I feel I do that far too seldom...

The thing why I stick with the 10k pots is that my friend want's to keep the amp as original as possible...

I did some calculations, installed some series resistance and changed the tail resistors also.
I now have it wired like this. (schem below)
It gives me a fairly good range (about 10volts) and now the bias is somewhere like 60-62% of maximum Pa when bias pots are halfway on the dial.

The first schem was straight from the Mark Huss Hiwatt page. Wonder why he didn't draw any series resistor in the schem between the -vdc and pots?
And what is the correct English word for that resistor?
Please forgive me, English is not my native language so sometimes I don't know all the right terms.

And finally word about those 220k safety resistors. If I install them I probably have to scale those bias supply resistors again a bit but that's no big deal.

Will those resistors make bias pots act more interactive?

-T

[FUCHSAUDIO]

Interaction between the pots is more a function of the main B+ power supply. The bias pots always draw the same current off the bias supply. As you set each pair the main B+ will vary, so you have to go back and forth between the pairs to get all the tubes "happy". A one ohm resistor on each cathode will let you read current direct in Mv. 30mv=30 ma etc.

[Dingleberry]

Interaction between the pots is more a function of the main B+ power supply. The bias pots always draw the same current off the bias supply. As you set each pair the main B+ will vary, so you have to go back and forth between the pairs to get all the tubes "happy". A one ohm resistor on each cathode will let you read current direct in Mv. 30mv=30 ma etc.

I was just rereading Kirchoff's law

Got to fiddle with those pots. I have the TAD bias probe so it's an easy job.

Now this got me thinking that actually a bias balance pot in conjunction with bias voltage pot would be kind of practical.
You could first adjust the balance and then the overall voltage.
On the other hand who can really hear a few milliamps difference and they didn't match the power tubes back then and quite a lot good records were still made.
Thanks for your help.

-T

[FUCHSAUDIO]

Bias/balance would work, but I prefer a pot per pair. I actually prefer a pot per tube if space allows, but have fun !

[Dingleberry]

Bias/balance would work, but I prefer a pot per pair. I actually prefer a pot per tube if space allows, but have fun !

Got to try those bias tricks in some future projects.
I will just install those safety resistors and bias the hiwatt.
And have fun with it

-T

[Hellhammer]

The first schem was straight from the Mark Huss Hiwatt page. Wonder why he didn't draw any series resistor in the schem between the -vdc and pots?
-T

That schem is for a 100W amp isn't it?

[bb5000]

This is the bias circuit in the DR201 clone that I built last year. The bias range between 35-100V works with the 6xEL34's, there are individual pots for each tube. The amp also works with 4xKT88's, the bias range covers those too. 6550's would probably fry at 700 Vp.

The circuit is basically the voltage doubler with 6x50k pots in parallell and the 3K3/5W resistor that sets the low limit of the bias range.

[img:180:100]http://s5.postimage.org/vzeljgarn/bias_ckt.jpg[/img]

[img:180:120]http://s5.postimage.org/733sqthfn/P0004815.jpg[/img]

[img:180:120]http://s5.postimage.org/4xbae9x8j/P0004801.jpg[/img]