Recap job for a 1974 DR103

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Recap job for a 1974 DR103

Postby vaccastracca on Mon Jun 29, 2015 12:43 pm

Hello everyone.
My old 1974 DR103 needs a full set of new capacitors, because the original ones started leaking acid.
What are the best options?
I am currently undecided between F&T, ARS and TAD Gold Cap, even though I must point out that the first are the only caps whose origin is certain (made in Germany)...about the other two, I can't find any reliable source that can confirm where they are made...
I've been reading infos here and there and there are even people that seem to notice tonal differences depending on the brand, while other people say it's just caps and you shouldn't worry about tone but just consider reliability.
Of course I am concerned about the tone change...i don't like changes! :P
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Re: Recap job for a 1974 DR103

Postby OldSchoolDave on Mon Jun 29, 2015 3:38 pm

vaccastracca wrote:... there are even people that seem to notice tonal differences depending on the brand,


Do those people have the hearing range of bats ;) ?

While that's not strictly the TONE section of the amp, bad caps can negatively impact overall SOUND and RESPONSE.

In any case, I don't find a reason to look beyond F&T for my Hiwatts (unless they're currently out of stock). AFAIK, ARS and TAD are relabelers (similiar to Groove Tubes), so they buy to a specification from potentially more than one manufacturer (and Country of Origin).

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Re: Recap job for a 1974 DR103

Postby vaccastracca on Mon Jun 29, 2015 8:55 pm

Thank you OldSchoolDave! :-)
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Re: Recap job for a 1974 DR103

Postby LD50 on Mon Jun 29, 2015 9:01 pm

I went with F&T and am delighted with the amp. Totally quiet, no hum no hiss, tight and responsive, I have full BVA preamp and KT77s in the output. I had planned on selling it after a tune up but it is shaping up as a keeper (probably because my son has grabbed it as his bass amp pending buying an Orange although it is hard to imagine an Orange sounding any better. For comparison I have a Matamp GT100 here at the moment and personally I think the Hiwatt much nicer.
All bleeding stops.......eventually.
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Re: Recap job for a 1974 DR103

Postby vaccastracca on Mon Jun 29, 2015 9:57 pm

Another vote for the F&T!
Great! :-)
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Re: Recap job for a 1974 DR103

Postby vaccastracca on Tue Jun 30, 2015 12:21 pm

By the way, i was almost forgetting to ask: when you change the capacitors in the DR103, people often say they change also the small axial 100uF 63V.
What's your opinion about this?
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Re: Recap job for a 1974 DR103

Postby LD50 on Tue Jun 30, 2015 2:03 pm

That is the bias cap.
If you are using the amp regularly and want the bias circuit to run correctly yes it is a good idea. IIRC when I did mine I went to a 200v rating as the new caps of that size capacitance are very small and I wanted one the same size to sit on the turrects nicely. Mine has an adjustable bias pot as well.
If you are not too prissy about it (and save the old parts and a photograph of how it is now) I would implement the adjustable bias mod that you find on the Mark Huss site, or one of the dual bias circuits floating around in this section of the forum.
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Re: Recap job for a 1974 DR103

Postby vaccastracca on Tue Jun 30, 2015 3:18 pm

The amp will not be used frequently. Let's say once a week, or even less.
I've never really considered any bias mod because I generally like to keep things the closest to original as possible...
Does this mod alter the tone in any way?
(messing with the tone is always my biggest paranoia! :P )
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Re: Recap job for a 1974 DR103

Postby OldSchoolDave on Tue Jun 30, 2015 6:53 pm

vaccastracca wrote:Does this mod alter the tone in any way?


Only in the sense of it improving a brittle tone, should your amp be biased too cold (unlikely, with a vintage Hiwatt). If your amp is biased too hot, the life of your output tubes could be shortened.

Whether or not you add a trim pot, I would recommend replacing that bias capacitor as part of your cap job.

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Re: Recap job for a 1974 DR103

Postby vaccastracca on Tue Jun 30, 2015 7:06 pm

OldSchoolDave wrote:
Whether or not you add a trim pot, I would recommend replacing that bias capacitor as part of your cap job.

Dave


The place where I'm buying the F&T caps from has two options for the axial:
- TREC (100uF 350V) dimensions are 20x36mm and it costs 2 euros
- F&T (100uF 450V) 25x49mm costs 5 euros

They actually offer also a couple of choices with 100uF63V value, but they are really tiny and I wouldn't trust them in a Hiwatt!
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Re: Recap job for a 1974 DR103

Postby LD50 on Tue Jun 30, 2015 10:19 pm

Image

Image

Image
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Re: Recap job for a 1974 DR103

Postby vaccastracca on Wed Jul 01, 2015 7:02 am

Great! Thanks for the pics, LD50! The can capacitors look like the ones i just bought.
Your bias cap is really huge, compared to the original one....you say it's a 100uF 200V?
By the way, looking at your pics, i noticed a thing in mine that got me suspicious, here is a picture:
Image
upload immagini
That gold thing....does it look like a non-native one?
Because last time i had the amp re-tubed (a couple of years ago), the tech mentioned a modification that he made in order to prevent power tubes to heat too much, and he said he added a resistor....could it be what he was talking about?
(in fact, since this last retube-job, the amp didn't sound anymore like it used to...and I'm going to retube it again along with the recap)
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Re: Recap job for a 1974 DR103

Postby Vince95 on Wed Jul 01, 2015 7:49 am

Hello,

You need to do the bias modification seriously. There is no way that adding a resistor will reduce the heat. You need to increase the bias voltage.
Originaly Hiwatt were equiped with Mullard valves that could handle higher bias current. This is no longer the case with the JJ or other valves. unless you find NOS Mullard valves you need to modify your amp. This modification can be very dangerous (lethal) since high voltages are present (500V).

See the forum for this modification, there are many examples with pictures and schematics.
It can be done by you or a confirmed technician. Recaping the amp is easy compared to the bias mod.
two '76 Hiwatt DR103 (with bias)
1978 Hiwatt SA112 (with bias)
1986 Marshall 2203
1996 Fender Twin Amp
TAD JTM45 clone
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Re: Recap job for a 1974 DR103

Postby LD50 on Wed Jul 01, 2015 9:44 am

That is a 220uF cap, but I cannot see where it is going to!

I do not see a resistor anywhere on those pics, only thing I could imagine is a huge Ercol in the B+ line to drop voltages.

Vince is right tho' your amp will sound better and last longer with correctly biased output stage and with modern tubes it is even more important and adjustable bias i svery useful (unless your new tubes are precooked and will never need adjustment once set up (ie not realistic). Most will drift a little and need adjustment sometime.
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Re: Recap job for a 1974 DR103

Postby vaccastracca on Wed Jul 01, 2015 9:57 am

LD50 wrote:That is a 220uF cap, but I cannot see where it is going to!

I do not see a resistor anywhere on those pics, only thing I could imagine is a huge Ercol in the B+ line to drop voltages.

Vince is right tho' your amp will sound better and last longer with correctly biased output stage and with modern tubes it is even more important and adjustable bias i svery useful (unless your new tubes are precooked and will never need adjustment once set up (ie not realistic). Most will drift a little and need adjustment sometime.


Here is a closer look at that gold cap:
Image
free image upload

I was reading the DR103 schematics, and in that area I don't see any 220uF50V cap...
So, I really would like to figure out wether it is original or added...
I guess it's what that tech added, and I agree that it has no sense, and in fact I won't be going to that tech anymore.

About the bias mod, is there any page where it's clearly explained step by step?
I don't have any tech that knows about it here, so I would have to gather infos first...
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