vintageHIWATTconvention.com

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?

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

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

Thx a ton Mark - I appreciate it.

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.

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!

Awesome, thanks!

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/

Anyone?


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

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

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.

Sorry guys - much better pics here -

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

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.

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

--mark

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