How to Install Adjustable Bias on Hylight Era DR103

[Ted B]

If I were building a clone amp from scratch, I might consider a redesigned bias circuit (as per Mark's suggestions). With an original Hylight era amp, I can vouch for the solution I provided as being non-invasive, effective, and reliable.

Do not make the assumption that using original production tubes in an original DR103 automatically results in optimum bias. This isn't so with any vintage amp. Proper bias adjustment will deliver best sound quality and longevity from older tubes as with newer tubes.

In the '70s, replacement Mullard EL34s were ubiquitous. Today, I see little sense in running a set of expensive Mullards needlessly hot in an old DR103. As such I recommend this bias mod for both original production and modern tubes. My '74 DR103 sounds great and will probably never (again) be fitted with Mullards or Brimars. When buying modern tubes for vintage amps, I suggest buying from a source that will select a set for lower than average idle current draw.

As far as adjusting the bias yourself, without a proper bias probe, it cane be done, but I wouldn't recommend you go poking around in there. If you want to play it safe, I recommend turning the bias pot such that you get the greatest negative voltage between the bias lead and ground before installing the power tubes, and start from there. You'll almost certainly still be on the hot side around the maximum range of adjustment (about-35V), but it should keep you out of trouble. If you get any audible crossover distortion (an audible fizzy fade with quieter input signals) , you'll need to turn it back a little. Under normal operation the power tube plates should not be glowing.

[Hi Matt]

Thanks Ted for your detailed reply.
I'm still a bit confused on how I should proceed to adjust the bias. As I'm french, I'm not really familiar with the english technical words.
The mod seems simple enough to install myself but maybe I should speak with a tech here about biasing.
Thank you again.
Any other comment is very welcome.
Best wishes,
Matthias

[Hi Matt]

Ted,
Which power tubes are you able to use now?
I heard good things about older production Winged "C" and GTEL34M.
Would you recommend other tubes?

[fltundra]

Hello,
I had to chime in as i have been dealing with the Power tubes with my 78' dr103. Ted's mod only gave me an extra -6 to -7 volts at most and i was on my second set of winged c's and i still couldn't get them below about 75% dissipation. There's two
problems with these old amps, they don't like 120+ volts at the wall, and like well documented, the weak bias windings. Mark's bias board works wonderful, as i can run any el34 i want with up to -100 volts on tap. Problem two, at 120 volts heaters were running at 6.93 volts. "Yikes"! and plates were at 505v. So i ended up buying one of these and problems solved, APC LE600 Line-R 600VA Automatic Voltage Regulator. It has three settings 110, 120 and 127volts. I'm using 110 setting which is supposed to be for Jamaica. My voltages are 106.8 - 109.2 regulated, with the heaters now at 6.12v, plates 450v, bias -46v. It's better to be a tad low, (within 5%) then high on the heater voltages.
V5 30.2ma
V6 26.0ma
V7 29.1ma
V8 27.5ma
Also tried the 105 volt setting on the amp, but heaters were still around 6.77v.
Were running a silver Keely java boost and i really understand now why these are such sleeper amps.

[Ted B]

Thanks Ted for your detailed reply.
I'm still a bit confused on how I should proceed to adjust the bias. As I'm french, I'm not really familiar with the english technical words.
The mod seems simple enough to install myself but maybe I should speak with a tech here about biasing.

Yes Matthias, I suggest you find an experienced tube amp tech. Ask him to bias the amp and explain the procedure at the same time. It is always good to have a clear understanding of this. If you use the amp often you may want to get your own bias probe.

Ted,
Which power tubes are you able to use now? I heard good things about older production Winged "C" and GTEL34M.
Would you recommend other tubes?

I am using JJ EL34L presently, but I will probably go to Tung Sol EL34B next. Yes, the Winged "C" EL34s have a good reputation.

Hello,
I had to chime in as i have been dealing with the Power tubes with my 78' dr103. Ted's mod only gave me an extra -6 to -7 volts at most and i was on my second set of winged c's and i still couldn't get them below about 75% dissipation. There's two problems with these old amps, they don't like 120+ volts at the wall, and like well documented, the weak bias windings.

Yes, the easy bias mod I described can deliver the maximum voltage of the existing bias circuit and no more. I find this is usually good enough if the power tubes are selected for low idle current as I suggested. Even so, I have found that Hiwatts sound best to my ears when the power tubes are run hot (e.g. 75-80%).

As for other voltage considerations, it's usually the high screen voltage of many vintage amps that makes it hard on the power tubes. What is also important is with some vintage European amps, the 120V tap is actually calibrated for 110 or 115V. This can be a problem, because it will always cause the voltages inside the amp to be too high when this tap is used. The automatic voltage regulator may help, but it won't solve the problem. What does solve the problem is running the amp at 230-240V:

http://www.parts-express.com/pe/showdet ... er=120-810

With some vintage Euro amps, I choose to run them only at 230-240V for this reason. When I do, the internal voltages agree with the schematic, and the problems disappear.

[mhuss]

Here is an idea for North American folks with high mains voltages:
http://www.instructables.com/id/VIntage ... Equipment/

Read the Geofex article as well.

--mark

[fltundra]

Mark,
Yes, I've seen that. But you have to admit for 35 dollars, fused, switchable, and 5 amp capacity, the APC does the trick.

[mhuss]

I hadn't noticed it was so cheap. $40 at Newegg, and $53 for the 1200VA model. That looks like a good way to go.

--mark

[Hi Matt]

Hi again,
Thank you for your replies.
Here in France we've got 230V so I set my Hiwatt on 240V. Does it mean we have less problems here in Europe with vintage Hiwatts than in the US?
What kind of good measurements tools (bias probes, etc...) would would recommend?
Thank you in advance,
Matthias

[Ted B]

Here in France we've got 230V so I set my Hiwatt on 240V. Does it mean we have less problems here in Europe with vintage Hiwatts than in the US?

Correct. It means the voltages inside your amp should agree with the original circuit diagram.

What kind of good measurements tools (bias probes, etc...) would would recommend?

A simple bias probe/meter that is inexpensive and easy to use can be found here:
http://www.ebay.com/itm/Analog-Tube-Amp ... 43af44187a

This seller is in HK, but sends to France (or anywhere). I use a relatively expensive Alessandro bias probe, but it does basically the same thing.

You will also need a voltage meter (up to 600V DC).

[Hi Matt]

Hello again.
Thanks for the imput.
I made a quick search on bias probes an found the Weber Bias Rite and the Quadstage BiasPro quite interesting products. Any feedback on these?
Ted, with the bias mod you suggest, you still need to find matched tubes isn't it? I understand you canno't adjust the bias current of each tube seperatly. Can it be probrematic to have some bias current diffenrence between the tubes? (as it sometimes happens even even when you order "matched" tube).
Thank you in advance,
Matthias

[mhuss]

Well, recall that back in the day when these amps were built, the tube marketers had not yet invented the "matched pair" concept! When they built these amps, they just grabbed the next two (or four) EL34s out of the box and off we go.

That said, the tolerances and variance from tube to tube are certainly higher now, so it's a good idea to buy an matched set, but unless you're building a tweaky hi-fi amp, there's no need to worry about exact matching. A little mismatch gives the sound more color.

--mark

[Hi Matt]

Thanks Mark,
I'm still hesitating between modding my vintage Hiwatt with Ted's mod and leaving it the way it is. Are there rugged enough new production tubes for unmodded Hiwatts out there?
I heard about the Winged C and GT EL34M being good.
What about the TAD EL34B STR "mullard inspired tubes" or the Harma E34L STR which has 30W plate dissipation (didn't hear about any other EL34 able to dissipate more than 25W). Anyone of you tried those?
Thanks in advance for the input.
Matthias

[mhuss]

Well, in this case it's less about the power handling and more about the bias sensitivity. I have older =C= tubes in my old 103, and they are biased correctly. On the other hand, I've seen some NOS RFTs and Siemens glow like coals in a grille. I don't have any experience with the Harmas or TADs in a vintage Hiwatt, maybe someone else can comment.

--mark

[Dr.HI-TONE]

The TAD EL34B STR sounds good when biased properly, but I have yet to see any of the Chinese EL34B STR that can handle a really highly biased HIWATT.

I have had excellent experience with the GT EL34M in HIWATTs that red plate every other brand/type. =C=, JJ's, EL34B STR,