Bias and plate voltage measurements.....

[JeffreyC]

Hey guys - I finally got around to hooking up my bias rite and taking some measurements....

74 Hiwatt DR504

Here's what I've found. My question is - how do I interpret this and decide which pair of tubes I should be using for
optimum tone based on these measurements? Also, for the JJ's and RFT's - I suppose I'd need to install an ajustable
bias circuit to get these numbers where they should be. I guess I'm a little surprised at the range of difference between
all of these and so a bit confused. Thx for your help...


NOS Teslas
Milliamps - 29.5, 28.3 Plate voltage - 505, 507 Optimum (70% ?) 34.6

JJ EL34L's
Milliamps - 52.6, 50.4 Plate voltage - 483, 483 Optimum (70% ?) 36.2

RFT's
Milliamps - 47.9, 48.5 Plate voltage - 485, 486 Optimum (70% ?) 36

Per the chart at Weber, it seems only the Teslas are operating within the acceptable range.

http://www.webervst.com/tubes1/calcbias.htm
Am I interpreting that correctly?

[JeffreyC]

I'd appreciate some input from any of you guys that are more familiar with vintage Hiwatts
and how you run them, etc.... Mhuss? Clayton?

Thanks,

Jeffrey

[mhuss]

Weird -- I replied to this about 10 hours ago, but it never showed up here.

You're right, taking the 70% rule of thumb, an EL34 (25 watts max plate dissipation) should idle at about 17.5 watts.

So, using your numbers (averaged):
NOS Teslas - 29mA x 505 volts = 14.6 watts (a bit cold, but OK)

JJ EL34L's- 51mA x 483 volts = 24.6 watts (too hot)

RFT's - 48mA x 485 volts = 23.3 watts (too hot)

I suspect the latter two might get a bit red in the plate when the amp is cranked.

Note that most of the bias testers actually measure cathode current (which equals plate + screen current), so they tend to read about 10% higher then actual plate current.

The moral of the story is, if you have a stock Hiwatt with no bias adjustment, you need to choose your output tubes carefully. And not just "by brand," a set of JJs made this year might read differently than a set made a year ago.

I have had good luck with the SED =C= EL34s, they seem to bias pretty close to 'just right' fairly consistently.

Original Mullard XF2s do as well, but they're a bit more expensive and harder to find!

--mark

[JeffreyC]

Thx a ton Mark - I appreciate it.

[JeffreyC]

Anyone have some good pics of the adjustable bias circuit installed in a Hylight DR504???
Is it easy to install? Will I need to the voltage doubler?

Thanks.

I believe my amp is completely stock aside from the CAP job I did and someone replaced the
master vol pot. I will post some pics later.

[Dr.HI-TONE]

You will not need the doubler on a DR504. Mark and I are working on a simple bolt bias adjustment/testing unit for vintage HIWATTs. stay tuned!

[JeffreyC]

Awesome, thanks!

[JeffreyC]

Finally got around to taking some pics of my Hiwatt - would appreciate comments about
what you experts see - all stock? I did the cap job, so I know about that,

For those of you who have already installed an adjustable bias circuit in your DR504, can you post a pic?

Thanks.

http://www.spinningaroundthesun.com/ima ... %20DR-504/

[JeffreyC]

Anyone?


DR - you have an idea when those bias kits will be available?

[mhuss]

The power wiring is not original, obviously. The screen resistors and perhaps the output tube sockets have been replaced. Other then that (and the re-cap) it looks all original to me.

The bias kits should be available in a week or two.

--mark

[Dr.HI-TONE]

The pics are kinda fuzzy, were they taken with a cell phone, or was the flash disabled?

if taken with a digital camera, set for macro and be sure the flash is on.

[JeffreyC]

Sorry guys - much better pics here -

http://www.spinningaroundthesun.com/ima ... 04/Hiwatt/

[JeffreyC]

Any word on the bias kits yet? I've got 2 pair of Winged C's on the way, but I think ultimately I'd
like the flexibility of being able to experiment with different tubes.

[mhuss]

Mea culpa, I've been too busy to button this up. Let me redouble my efforts.

--mark

[JeffreyC]

Cool, thanks Mark. Will these be on the new items section of the VHR site?