Retubing corksniffing advice needed :-)

[theWalrus]

Hi Folks,

The high voltage fuse of my DR103 went and my tech says it's about time to change the tubes. I need re-tubing advice. Here is what I have in mind, after reading:

NOS power tubes: NOS Svetlana SED EL34 Winged C (4X) with "burn in" for matching. They seem like a good option.

NOS preamp tubes:
(a) two separate matched pairs of Mullard ECC83
(b) two separate matched pairs of Valvo ECC83 with 45 degree getters. The getters seem like a good thing.
(c) all 4 matched pairs of Mullard or Valvo ECC83 --- worth the ludicrous prices? I actually found both of those on ebay, but I got outbid on the last seconds; sometimes it's good not to get something you want!

Now, the laughable part: do these really matter for a good reason or it's pure audiophile corksniffing:

- same factory plant?
- same matching IDs?
- whether not all 4 tubes are "matched"?

Maybe, for a technical reason, a mix of different tube designs is better for different locations in a Hiwatt; I don't know amps' inner workings in great depth.

Thanks and best regards!

[OldSchoolDave]

I haven't quite delved into that depth of corksniffing yet, so I can't say whether the cost would be worth it or not. I will say:

1st of all, don't get hung up on "matched" preamp tubes. That's audiophile hype. There's even a question as to how close "matched" power tubes really are at operational voltages. Some folks even prefer a slight mismatch in their power section...

Get a really nice, strong Mullard ECC83 for V1 and an equally nice ECC81/CV4024 for V4/PI.

Keep in mind that one half of V2 doesn't get used in the standard Hiwatt circuit so, in theory, you could have an ECC83 with a dead side in it (providing it's the right one) and be just fine. In that case, a "matched" tube would be completely unnecessary.

V3 is probably the least critical tube. I typically don't "waste" a vintage tube in that socket, but it's your money !

Dave

[theWalrus]

Thanks Dave. That was useful.

[mhuss]

I agree with Dave,

--mark

[theWalrus]

So... I've finally heard from my tech on what happened with my DR103. The HV fuse went and I just need new tubes and electrolytic caps. He made a couple of suggestions:

- One problem is that the mains in the US have changed from ~110 to ~ 124V over the years. Hiwatts deliver extremely high voltages, and that difference in mains gets amplified. He suggests putting a voltage doubler circuit so he can bias the tubes just right. I don't think that's a good idea because it's a mod, although he says he'd be able to bias the tubes better this way.

- The other suggestion was to use the Electro-Harmonix 6CA7 power tubes, because they can handle high voltage better than, say a NOS =C= EL34 power tube, according to him. They seem to be a copy of an old Phillips 6CA7 tetrode.

Would you guys recommend these 6CA7 in favor of NOS =C= EL34? I'm not a tube expert.

[Dr.HI-TONE]

I would opt for the voltage doubler in this case

[theWalrus]

I don't know, Doc. It sounds good to have a voltage doubler from a technical standpoint, but it's a "mod". When I hear or think of that word, I think of Big Bob...

https://www.youtube.com/watch?v=AGy0pPllHVQ

skip to 6:03

The only mod on my amp is the power cable for US outlet use, but that's it; I don't see any harm. I guess even if it may not sound as good as it potentially could, it would keep it authentic to what a DR103 is by design.

A zero-mod alternative, in my opinion, would be to get a Variac and set it at 110 or 115V. This is what I found:

http://variac.com/staco_variable_transformer_3pn221.htm

[Dr.HI-TONE]

the 120+v. coming out of the wall outlet is not the issue.
this is a common issue with some batches of partridge transformers in the 1970's, they simply did not provide enough bias voltage.

the bias doubler is your best option.

[theWalrus]

The way you put it, it sounds like it has everything to do with the transformers and it was a production issue.

I heard about the voltage doubler when he first glanced at the amp's guts and pointed to a very specific resistor, mentioning a previous DR103 he had worked on previously; it wasn't based on a test. The amp was just tested for functionality with old tubes and it seems fine.

I'm thinking of just having a bias set for as good as possible without making changes. It will cost me more to have the voltage doubling and I bet that depreciates the re-sale value. The worst that can happen, I guess, is a shorter power tube lifetime.

[mhuss]

The problem is that the fixed bias in the 70s amps was set for the Mullard EL34s available at that time. The tubes being made these days require a more negative bias voltage to idle at the right current. If you reduce the mains voltage, it will decrease the B+ but it will also decrease the bias voltage.

You might want to try increasing the 47k bleeder in the bias supply to something much larger, e.g., 470k. This will decrease the load on the bias winding and will increase the voltage a bit. Plus it's a one-resistor change, easily reversible.

--mark

[theWalrus]

I see; that makes sense, Mark. I'll discuss further with the tech and bring up your comment.

It also surprised me that a Variac seems unnecessary despite the fact that it is a vintage amp.

[E-Rock]

How about using a simple step-up transformer to deliver ~240v to the amp (and obviously using the amp's 245v tap)? Been doing this for quite some time with my '69 SC MKIII with no issues…in fact, the amp actually sounds "sweeter" to me @ 245v vs. the 115v setting. Would this help with this DR103?

[Dr.HI-TONE]

No, it will not change the bias voltage that the PT is creating.
Some of these from the early 1970's make the tubes run considerably hotter than others.

[mhuss]

In a perfect world, a Variac set for whatever sets the heater voltage to precisely 6.3 VAC is ideal (for tube life, anyway). However, you'd have to have a semi-permanent meter fitted and regularly change the Variac to suit the current mains voltage, which varies a bit depending on local demand, etc.

In Hiwatts, there's plenty of leeway on all the filter caps, so really it's only the tube life you're extending by reducing the mains voltage.

If you have the patience, money and wherewithal, setting the Variac output to 115 (or whatever the amp says it wants) when your local mains is at mean value -- not the highest or lowest value -- would be an improvement (again, for the tube heater life).

FWIW, I actually built a 'vintage voltage' adapter that reduces the input mains by 6 or 12VAC, but I almost never use it except when working on fragile old Valco amps and their relatives.

--mark

[theWalrus]

I heard my DR103 for the first time in months! Man, I missed this amp.

I went down the cork-sniffing route and got F&T caps from Hitone, NOS (Blackburn) Mullard preamp [ECC83's for V1 and V2 and ECC81 for V4], a Chinese one for V3, and Mullard NOS Xf2 EL34 power tubes. Plugged in yesterday, and it sounds phenomenal! I spent over 3 hours straight with it.

- The biasing went well with just a resistor change, as Mark suggested!

- Saved an NOS ECC83 (in V3) instead of putting all NOS ECC83/83/83/81, as Dave suggested.

- 22nd high-e fret bends are now audible. I could not hear them before. Clayton helped me figure that one out. Needed re-cap. My tech concurs with that.

- It sounds like a hi-fi blasting beast; the notes are well defined and "3D" even cranked up which is how I play it. Perfect for blues.

I'll post sound clips or a video when I get a chance.

Friends, I just wanted to say thank you