70s Orange clone with high plate/screen voltage

Completed amps from Fender, Orange, Hiwatt, Vox, etc.

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David_Ageitos
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70s Orange clone with high plate/screen voltage

Post by David_Ageitos » Mon May 08, 2017 7:01 am

Hi there,
this is my first post (high everybody).

Last month i get an old amp made in Barcelona(Spain) in mid 70s. Is a close copy of a early Orange/Matamp amp.

What worries to me is the high screen voltage and the plate to plate load it uses.
550vdc and 1k8 with 4xEL34

I read a lot about similar amps and found this topic in this same forum:
viewtopic.php?f=33&t=44023&start=450

If my english level is right i understand that a easy solution will be attach the screen supply to power transformer center tap, that is at half voltage of main B+, connected to middle point of e-caps.

In the same topic they ask for a higher plate to plate load. I can use the 8ohm tap with a 16 ohm speaker for a 3k6 load.

I attached the original schematic and a modded one (V2)

https://goo.gl/photos/oLp9SkjMnb8eQh9r7

Any correction?
Last edited by David_Ageitos on Tue May 09, 2017 4:07 pm, edited 1 time in total.

daveweyer
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Re: 70s Orange clone with high plate/screen voltage

Post by daveweyer » Tue May 09, 2017 11:48 am

That's the right idea. Either plate load will work, but you'll be able to run the tubes harder with the 1.8K load. See how you like the sound each way.

David_Ageitos
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Re: 70s Orange clone with high plate/screen voltage

Post by David_Ageitos » Tue May 09, 2017 12:17 pm

Thanks!!!

One more question.
Using a higher load than original (3k6 vs 1k8), increase the current demand in the O.T.?

Transformers are big in this amp, but in the past i had a smooking transformer using a 16ohm speaker in a 8ohm secondary.

Thanks again!

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neikeel
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Re: 70s Orange clone with high plate/screen voltage

Post by neikeel » Mon May 22, 2017 4:53 am

I am building an ORST50 clone at the moment.

The original I have has 180v-0-180vac HT using the centre tap and voltage doubler with bridge rectifier for approx. 510vdc on the plates. So as per your schem 560v is likely for your PT.

The Partridge I have is from a Sound City or Carslboro with no centre tap and will give me 450v rectified.

You can use bigger screen resistors 2k2 or higher to get the screen voltages down or a couple of Zeners in series to ground to give the tubes an easier time.

I am using a Partridge 100w (1750) OT for my 50w but an going to use the 8ohm tap as 16 etc on the hook up as I am only using x2 EL34s.

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Neil

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