Music Man RD100 conversion

[Dingleberry]

Hi.

I have a MM RD100 lying around. It has been sitting in my workshop for about 10 years now.
OT broken.
I've been reading this forum a lot for past two years and noticed that HAD made some Dumbles from Music Man amps.

I was thinking that maybe I can do it aswell. I have been fixing and building amps for about ten years now.
I actually have probably a good candidate for output transformer. It's from Dynacord Eminent 2T model amp.
It run 2xEL34's and with plate voltages something like 750V. OT is rated for 80W and it's HUGE.

The main concern is Can the Music man PT supply enough current for pre amp tubes?
There is only power tubes in RD100.
Do I have to find a new PT also?

-T

[ToneMerc]

T, HAD pulled the MusicMan iron and used them as donors for the 700V plate/350V screen supply in his own builds. If you wanted to do a conversion, one common method is to strip the entire chassis down leaving only the power and output transformer and then build it back with your own voltage doubler power, bias, relay supply and preamp boards.

Also, can't just use any off the shelf OT, you have to use a OT that has an input impedence load that's compatible with plate voltages around 700V. If your existing OT is bad, the easiest thing to to is to get a replacement from Mojo Supply. When I build my ODS with a 700V supply I used a replacement MusicMan PT and just had Heyboer wind me a compatible OT with 4, 8 & 16 taps.

Since they didn't have preamp tubes the 100W PT is only rated for around 6A on the filament winding. My PT runs slightly warm with 4 EL34's and 3 12AX7's hanging off of it.

http://www.mojotone.com/amp-parts/tranf ... ransformer



Here's a link to a RD50 conversion


https://tubeamparchive.com/viewtopic.ph ... light=rd50

TM

[talbany]

Hi.

I have a MM RD100 lying around. It has been sitting in my workshop for about 10 years now.
OT broken.
I've been reading this forum a lot for past two years and noticed that HAD made some Dumbles from Music Man amps.

I was thinking that maybe I can do it aswell. I have been fixing and building amps for about ten years now.
I actually have probably a good candidate for output transformer. It's from Dynacord Eminent 2T model amp.
It run 2xEL34's and with plate voltages something like 750V. OT is rated for 80W and it's HUGE.

The main concern is Can the Music man PT supply enough current for pre amp tubes?
There is only power tubes in RD100.
Do I have to find a new PT also?

-T

T.
What are the numbers stamped on the MM PT..Something like 220-130 etc..

Tony

[Dingleberry]

The Dynacord OT is made for 700V use. Primary impedance is correct with that high voltages. It has 4, 8, and 16 ohm taps. The Dynacord Eminent 2T schematic shows that. While I get home I can send the schematic attached. Only concern was is that RD100 PT cabable to run 2xEL34 and 4x12AX7. Couldn't find any specs for those trannies, but I assume it is.

-T

[Dingleberry]

I'll check the PT code when I get back to workshop.

-T

[talbany]

The Dynacord OT is made for 700V use. Primary impedance is correct with that high voltages. It has 4, 8, and 16 ohm taps. The Dynacord Eminent 2T schematic shows that. While I get home I can send the schematic attached. Only concern was is that RD100 PT cabable to run 2xEL34 and 4x12AX7. Couldn't find any specs for those trannies, but I assume it is.

-T

Worked on one of these a while back..
The DCN 307 would certainly work..The question is how will it sound in a D-Style amp?
Chances are the MM PT will work no problem..

[Cliff Schecht]

So what is the primary impedance of the MusicMan OT that is appropriate for such high plate voltages? Is it around 3k (seems about right for a quad of 34's with high plates)?

[martin manning]

I'd say not less than 4k9 for Va 700V and Vg2 350V for 4x EL34.

[talbany]

+1 Martin

130W Musicman OPT... Output primary winding resistance is 220 ohms (plate to plate), leakage inductance is 33mH, and the transformer ratio is 24.25:1. This gives a primary impedance of 4,700 ohms (8 * 24.25²). Plate supply is 695V and screen supply is 347V, and the amp runs with 24mA bias current per valve (4 x EL34). It gives a clean 120W, equivalent to 60W from a pair of valves with a 9,400 ohm plate to plate primary impedance and the same supply voltages.

Hope This Helps!!

Tony

[Cliff Schecht]

Makes sense, I was really just taking a stab in the dark (didn't run any numbers). Higher plate voltage requires a larger impedance to drop the left side of the load line (the part that hits the plate current on the Y-axis) down below the max power dissipation line..

Ugh, it's already been too long since I've run numbers for tubes, I'm getting rusty! Living in the BiCMOS world at work and school has really thrown off my tube game .

Maybe someday I'll find the cheap Peavey or MusicMan four banger that I've been looking for forever to convert to some sort of Dumble style head. I'm not sure if it's better to find the amp first and then decide on a schematic or the other way around but my typical build style predicts the former, not the latter. I'll start that thread when the time comes lol..

[ToneMerc]

So what is the primary impedance of the MusicMan OT that is appropriate for such high plate voltages? Is it around 3k (seems about right for a quad of 34's with high plates)?

The stock replacement MusicMan 100W OT input is 5.5K. I had Heyboer wind me a Fender Twin OT @ 5.5K with 4, 8, 16 taps, it works great with 710V on the plates.

TM

[Dingleberry]

Thanks for replies.
I measured the Dynacord OT a while ago and that high plate voltage explains the impedance ratio of that transformer.
I measured it from all three different taps using
signal generator/volt meter method and got
"close enough to boogie" results.
4 ohm tap ratio was 2209:1
8 ohm tap ratio was 1075:1
16 ohm tap ratio was 518:1

So that would make, depending on the secondary impedance chosen,
the primary impedance something like 8k2 - 8k8.

It's from Eminent 2T, the" T" in the end meaning "transistor" i.e. the amp has a solid state pre amp.
Here are couple of pics attached, few just showing the size difference between the Dynacord OT and the stock Music Man OT.

And Tony, the numbers stamped on the MM PT are:
200-130-250
Can you encrypt it? Couldn't find any info on the current available.
Only some info on voltages...

-T

[talbany]

Dingleberry

Are you overseas.. 200-130-250 shows export transformer..100w,150w. @ 220 250v AC..
If you are? that transformer should be more than adequate to run a 50W (Since I assume you are planning to run w/Dynacord OPT)
Let me know if you need the wiring diagram..

you gotta do something about your username .. ..Kidding

Tony

[DonMoose]

Would it be a terrible thing to uprate the relay supply tranny a bit and use that for the preamp heaters as well?

3 x 300mA is pushing that winding a little. 4 x 300mA would really worry me.

[Dingleberry]

Hi Tony.
I live in Finland. Downloaded the amp schematics and that transformer hookup schemo from the Ernie Ball site. It's an export model.
Just wasn't sure of the current available, but if they used the same PT for those higher power (4xEL34) MM amps, it should handle 2xEL34 and 3 pre amp tubes with no problem.
Yes I'm gonna use the Dynacord OT for that project.
It's gonna be a tight fit thought. Not too much space in that chassis...
The hardest thing will probably be to decide which model to build...

-T