1987 - 50W master vol with 400V plate voltage?

[dr.barlo]

Hi guys,

First post here.

I am quite familiar with 1987's and love them. In mid 90's when I was playing in local clubs, I owned/used 2 of them (not together tho) until they broke. One was the OT, the other something to do with its caps. Wish there was a forum like this one in those days. Anyways, I don't know much about those ones that I had, least of all their plate voltage. I think they were from mid 70's.

Recently, I am involved in a 1987 clone with master vol and RR mod, that will be built by a buddy of mine, and because of a change of heart (nevermind the details), the clone would be done with the same parts corresponding to a 68 Plexi, but with 400V plate voltage. We have almost all the parts including the PT, and the PT that we have cannot deliver more than this level.

So what I wanted to ask was: What should I expect? Could you give some feedback? Would it break too early?

Obviously, don't know much about the technical details, and their effects on the overall performance of the amplifier. Should I be scared? Get a more powerful PT?

Thanks in advance.

Best,

B

[neikeel]

I built one of these (1987 with RR mod and PPIMV) I used a Marstran -164 PT that gave around 395v of B+.

It worked out very well and the owner is very pleased:









Oh yes and he wanted purple

[dr.barlo]

Thanks neikeel,

Looks wonderful!

B

[demonufo]

Oh yes and he wanted purple

That's because he obviously has impeccable taste!

I take it you'd ran out of .68 mustards at that time?

The last 1987 I saw that had a plate voltage around the 390 mark, sounded wonderful. Certainly a tad more compressed than what I was used to, although admittedly I don't get to see many non-masters, and I'm more of a master vol guy myself.

Two other 1987's that I see from time to time running around the 460-470 mark seem to have a little more headroom admittedly, but not quite as fluid on lead runs as the one with the lower plate voltage.

I've no idea whether or not this correllates with anybody else's findings however.

[dr.barlo]

Thanks for sharing.

Now, I am relieved. Appreciate it guys.

B

[Roe]

alot of the 50watters run at low voltages. but personally, I leave out the 10k voltage drop resistor if the voltage is low so that only one 8k2 is left btw screens and pi

[PCollen]

I have a JTM45 running KT66's that I've split the cathodes on (1.5K/22uF and 2.7K/0.68uF), put a 4.7uF/35Vdc cap across V2a cathode resistor, and changed the PI couping caps to .022uF. So, it's kind of a half-breed.

I run 405 plate voltage, 404 on the screens, and swapped the 8.2K rail resistor for a 15K, dropping PI voltage supply to about 310 Vdc. Two more DC p/s sections, seperated by 10K's give V2 supply at about
280, and V1 supply at about 260. ( Note: All voltages depend, of course, on todays wall voltages courtesy of Florida Power and Light). I bias the KT66's at 70%.

The amp sounds GLORIOUS (angelic trumpets resonding on high) and has a lot of 'bounce' to it compared to higher voltages. Breakup does occur earlier than with higher voltages, although there is still plenty of clean headroom particularly using a Strat. And it's still flippin' LOUD, through my Avater 2 x 12 with HELLAtones (they're hella-sweet).

From what I know, it was common with 50 watt Marshalls in the 70's to have about 400 Vdc on the plates, and correspondingly lower voltages thorughout the amps. Look at the two 50 W amp charts here:

http://www.kbapps.com/audio/schematics/ ... chart.html" onclick="window.open(this.href);return false;