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Marshalls...

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Re: Marshalls...

PostPosted: Wed Aug 26, 2020 2:27 pm
by Sam Spoons
My old MusicMan RP65/100 produced 100 watts before clipping from a pair of EL34s, the only difference between the two versions was the transformers, the tranny in the 65 watt supplied 250V HT and the one in the 100 watt, 350V. I swapped the trannies* as I wanted the power but didn't like the speaker in the more powerful version.

* I worked in MI retail, the importers were kind enough to send me a pair of the bigger trannies and take the originals back into spares stock for, IIRC, £25.

Re: Marshalls...

PostPosted: Wed Aug 26, 2020 2:56 pm
by ef37a
Sam Spoons wrote:My old MusicMan RP65/100 produced 100 watts before clipping from a pair of EL34s, the only difference between the two versions was the transformers, the tranny in the 65 watt supplied 250V HT and the one in the 100 watt, 350V. I swapped the trannies* as I wanted the power but didn't like the speaker in the more powerful version.

* I worked in MI retail, the importers were kind enough to send me a pair of the bigger trannies and take the originals back into spares stock for, IIRC, £25.

Yes Sam 'The Book' says you can get 100W from a pair of EL34s but I don't know of any amplifier manufacturer that has ever marketed such an amp? 100 watters are usually 4 25W bottles.

In fact I was once asked to test a couple of amp to see what they could put out for 10% THD?
An Artisan 100 easily gave 120 watts and the HT-60 stage topped 80W for just two EL34s. So, it would be quite in order to market those models as "Artisan 120" and "Stage 80" because 10% THD is the rating point in most valve data specifications. (in these days of 3 or 4 decimal places for converter distortion, 10% sounds horrendous but in fact, for a single source it is on the verge of detectability and a modest 'rock edge' is much higher)

Note as well, the tests were fairly done at the rated 230V input. Most of us get 240V and so you could probably stick at least another 5W atop those figures.

Dave.

Re: Marshalls...

PostPosted: Wed Aug 26, 2020 4:12 pm
by Sam Spoons
MusicMan did market the RP/RD100 as a 100 watt amp with only two EL34's, we didn't believe it so that was why we went to the trouble of measuring it.

We got a fraction over 100 watts measured for a 1kHz sine wave into an 8 ohm dummy load. We wound it up until the scope showed the top of the waveform flattening and then backed it off a touch. We didn't have the facilities to measure the THD but our method was accurate enough for me.

Re: Marshalls...

PostPosted: Wed Aug 26, 2020 6:17 pm
by Hewesy
Interestingly some Marshall users did fit the KT88 - or 6550 which I think is interchangeable with the KT?

Zakk Wylde uses them in his Marshall's, i think possibly Randy Rhodes did too and a few others (Billy Corgan?).

Hewesy

Re: Marshalls...

PostPosted: Wed Aug 26, 2020 8:23 pm
by ef37a
Hewesy wrote:Interestingly some Marshall users did fit the KT88 - or 6550 which I think is interchangeable with the KT?

Zakk Wylde uses them in his Marshall's, i think possibly Randy Rhodes did too and a few others (Billy Corgan?).

Hewesy

GREAT debate found in the audiophool community as to whether or not they are exact equivalents. In the 'Ball of chalk and String' design world of guitar amps? They are the same but like any power valve change, bias must be checked.

Sam, in my opinion 'watts (well V across R) at a specified distortion level is the only fair way to specify amplifiers. You said yourself "observed clipping then backed it off a bit" !! That is a subjective judgement. Of course, 100 watts or 95 (at X%THD) matters not at all to player or punter, both too B loud but some people get sniffy when an amp does not meet a published (or assumed ) power specification.

No distortion meter? Wine into water to make for this application. J L Hood published a design based on a parallel T filter and SINGLE transistor! Today we would use an op amp.
Reading THD at 1% and higher is easy especially when you have tens of volts of signal. It is measuring to 3 decimal places where you need the Docs AP rig!

BTW anyone here still using tape seriously would do well to knock up the JLH meter circuit.

Dave.

Re: Marshalls...

PostPosted: Thu Aug 27, 2020 8:37 am
by Kwackman
ef37a wrote:anyone here still using tape seriously would do well to knock up the JLH meter circuit.

But in that situation it wouldn't be measuring distortion, but "Analogue Warmth"... ;) :mrgreen:

Re: Marshalls...

PostPosted: Thu Aug 27, 2020 8:47 am
by ef37a
Kwackman wrote:
ef37a wrote:anyone here still using tape seriously would do well to knock up the JLH meter circuit.

But in that situation it wouldn't be measuring distortion, but "Analogue Warmth"... ;) :mrgreen:

Yup, THREE parts in a hundred of the bugger!

Oh and, I got it wrongly, the JLH distortion meter is a somewhat more elaborate beast based on Wein networks. I shall try to find the simple 'bootstrapped' Pll T circuit, it worked well. Maybe it was in Studio Sound? (all a very long time ago........)

Dave

Re: Marshalls...

PostPosted: Thu Aug 27, 2020 12:26 pm
by Sam Spoons
All that was a very long time ago, and the amp was sold long ago (something I went on to regret) but yes, it was loud.