Hiwatt dr504 bias problem

smokey · 07-30-2008, 12:44 PM

Hi there,
I have an Hiwatt 50w dr504 amp with an output tube glowing cherry red. The bias voltage is -39vdc and I measured 70mA across a 1ohm resistor on V6. B+ is 460vdc and everything else looks correct. I've switched for different brands of el34 with various results but all exceed the normal range.
Any ideas?
Thanks

MWJB · 07-30-2008, 12:53 PM

install a cermet pot wired as a variable resistor between the bias diode and the grounded 27K resistor in the bias supply.

You need to increase the negative bias voltage.

smokey · 07-30-2008, 01:02 PM

MWJB, thanks for your prompt answer, I know I can install a bias pot but I would like to understand why I get twice the normal amount of current for a difference of only -3v in respect of the -36v stock voltage.
thanks

MWJB · 07-30-2008, 01:07 PM

Assuming that everything is working as it should (like that 1ohm resistor is tied to the cathode of one tube only & and has a good ground reference) plate current is read in mA, not negative voltage. At a constant negative voltage the plate current will vary wildly from tube to tube & brand to brand.

-39v doesn't sound like nearly enough to me, for a plate voltage of 460v.

smokey · 07-30-2008, 01:22 PM

The best plate current reading I get is 52mA when used with JJ's E34l. Tubes don't glow red and the amp sounds ok. With a plate dissipation of 23w do you think It's safe to run an amp like that?
thanks

MWJB · 07-30-2008, 01:27 PM

JJ still only rate this tube at 25W, mid thirties may be safer (note that your plate voltage will rise as the curent comes down, so monitor both...as well a tone of course).

smokey · 07-30-2008, 01:43 PM

Would you please have a schematic for installing a bias pot?
thanks

MWJB · 07-30-2008, 02:03 PM

I'm sorry, I don't.

If you find the main rectifier diodes, there will be 2 wires from the PT feeding them, one wire after the diodes that carries rectified, dc B+ voltage. There will also be a 4th wire from the PT side of one diode that then feeds a 110K 1W resistor. Following the 110K is a backwards diode (stripe towards the PT), the other end of which is connected to a 27K resistor, an electrolytic cap (+ to ground) and the wire that feeds the resistors at pin 5 of the E34Ls.

Remove the 27K resistor, without disturbing anything else. Get a single turn cermet pot of between 50K & 20K (see hoffmanamps.com). Solder the 27K resistor from the bottom RH leg of the pot, this resistor goes to ground. The top middle leg of the pot goes to the bias diode, the electrolytic cap & the wire feeding the resistors at pin 5 of your E34Ls.

Make sure all is sturdily mounted (if necessary dress the pot legs so that they sit flat, but raised a little from, the bias board and use a dab of silicon cement to secure the pot) and that there are no shorts, especially from the unused leg of the cermet pot.

Remove the power tubes, power up & set negative voltage to -50 to -55vdc, power down & re-install the tubes. Now set plate current to 30-35mA read accross the 1ohm resistor.

Enzo · 07-30-2008, 02:32 PM

My concern here is that ONE tube was red plating, not all of them. Were they all the same type and brand? Did you move those tubes around to see if the same tube red plated no matter where it was, and other tubes did not in the same socket?

I'd agree the bias seems low, but let's not just up and modify the amp if the real problem is a bad tube. Figure out what the problem is and THEN start to modify things.

smokey · 07-30-2008, 02:43 PM

Hi Enzo,
Sorry I meant both tubes went red.

Enzo · 07-30-2008, 04:58 PM

Oh...
Then never mind...

07-30-2008, 12:44 PM	#1
smokey Junior Member Join Date: Aug 2007 Posts: 18	Hiwatt dr504 bias problem Hi there, I have an Hiwatt 50w dr504 amp with an output tube glowing cherry red. The bias voltage is -39vdc and I measured 70mA across a 1ohm resistor on V6. B+ is 460vdc and everything else looks correct. I've switched for different brands of el34 with various results but all exceed the normal range. Any ideas? Thanks

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07-30-2008, 12:53 PM	#2
MWJB Old Timer Join Date: Jun 2006 Posts: 1,920	install a cermet pot wired as a variable resistor between the bias diode and the grounded 27K resistor in the bias supply. You need to increase the negative bias voltage.

07-30-2008, 01:02 PM	#3
smokey Junior Member Join Date: Aug 2007 Posts: 18	MWJB, thanks for your prompt answer, I know I can install a bias pot but I would like to understand why I get twice the normal amount of current for a difference of only -3v in respect of the -36v stock voltage. thanks

07-30-2008, 01:07 PM	#4
MWJB Old Timer Join Date: Jun 2006 Posts: 1,920	Assuming that everything is working as it should (like that 1ohm resistor is tied to the cathode of one tube only & and has a good ground reference) plate current is read in mA, not negative voltage. At a constant negative voltage the plate current will vary wildly from tube to tube & brand to brand. -39v doesn't sound like nearly enough to me, for a plate voltage of 460v.

07-30-2008, 01:22 PM	#5
smokey Junior Member Join Date: Aug 2007 Posts: 18	The best plate current reading I get is 52mA when used with JJ's E34l. Tubes don't glow red and the amp sounds ok. With a plate dissipation of 23w do you think It's safe to run an amp like that? thanks

07-30-2008, 01:27 PM	#6
MWJB Old Timer Join Date: Jun 2006 Posts: 1,920	JJ still only rate this tube at 25W, mid thirties may be safer (note that your plate voltage will rise as the curent comes down, so monitor both...as well a tone of course).

07-30-2008, 01:43 PM	#7
smokey Junior Member Join Date: Aug 2007 Posts: 18	Would you please have a schematic for installing a bias pot? thanks

07-30-2008, 02:03 PM	#8
MWJB Old Timer Join Date: Jun 2006 Posts: 1,920	I'm sorry, I don't. If you find the main rectifier diodes, there will be 2 wires from the PT feeding them, one wire after the diodes that carries rectified, dc B+ voltage. There will also be a 4th wire from the PT side of one diode that then feeds a 110K 1W resistor. Following the 110K is a backwards diode (stripe towards the PT), the other end of which is connected to a 27K resistor, an electrolytic cap (+ to ground) and the wire that feeds the resistors at pin 5 of the E34Ls. Remove the 27K resistor, without disturbing anything else. Get a single turn cermet pot of between 50K & 20K (see hoffmanamps.com). Solder the 27K resistor from the bottom RH leg of the pot, this resistor goes to ground. The top middle leg of the pot goes to the bias diode, the electrolytic cap & the wire feeding the resistors at pin 5 of your E34Ls. Make sure all is sturdily mounted (if necessary dress the pot legs so that they sit flat, but raised a little from, the bias board and use a dab of silicon cement to secure the pot) and that there are no shorts, especially from the unused leg of the cermet pot. Remove the power tubes, power up & set negative voltage to -50 to -55vdc, power down & re-install the tubes. Now set plate current to 30-35mA read accross the 1ohm resistor.

07-30-2008, 02:32 PM	#9
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,287	My concern here is that ONE tube was red plating, not all of them. Were they all the same type and brand? Did you move those tubes around to see if the same tube red plated no matter where it was, and other tubes did not in the same socket? I'd agree the bias seems low, but let's not just up and modify the amp if the real problem is a bad tube. Figure out what the problem is and THEN start to modify things.

07-30-2008, 02:43 PM	#10
smokey Junior Member Join Date: Aug 2007 Posts: 18	Hi Enzo, Sorry I meant both tubes went red.

07-30-2008, 04:58 PM	#11
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,287	Oh... Then never mind...

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