Rockmaster Preamp: General modding questions

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dicianne
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Rockmaster Preamp: General modding questions

Post by dicianne » Sat Aug 28, 2021 2:08 am

So I'm beginning to mod my working rockmaster preamp. I don't run into much of a problem when it comes to identifying and replacing the right resistors. They're easier to figure out than capacitor types for me. I''m not sure which capacitors are the yellow ones. Are these axial film, or axial tantalum?
I presume whoever designed this had these specific capacitors in mind. I only have regular film and aluminum caps, and tiny ceramic caps in the 2000pf-5000pf range needed for the EQ mods. Also, do I have to match voltage too?
I have no idea what type of capacitor c14 is (the white one), C9 looks like a resistor, but I'm aware caps can be color coded too.
At any rate, can I replace all of these with the caps below?? I'd rather do this weekend than wait another week for a mouser order with the right caps
Thanks!
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Enzo
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Re: Rockmaster Preamp: General modding questions

Post by Enzo » Sat Aug 28, 2021 9:58 am

Well, caps are caps, pretty much. If your mod instructions specify a type (assuming there is a reason to specify) it doesn't matter what the old type was.

SMall ceramic caps can look like resistors.

Tantalums? Are there tants in this? I never use them. They have zero tolerance for reverse voltage and cannot handle over-voltage worth a damn. Use regular electrolytics for them.

You don't have to "match" voltages, but if the old cap was 400v, it was 400v for a reason, so don't put a 50v in its place. However if the old one was 50v, and all you have is 400v, that is fine. You can always use higher voltage caps, as long as they fit the space.

What are they asking you to install in C14? The schematic says it is a 0.001uf 630v cap. It has radial leads, but you could use axial too. There is no reason to change C14 unless to change value, so...?

dicianne
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Re: Rockmaster Preamp: General modding questions

Post by dicianne » Sat Aug 28, 2021 6:33 pm

I'm only asking what they are, because unlike the schematics of the ultra series (similar to the rockmaster), the schematics of the rockmaster doesn't come with a part list. For example C14 in the Ultra 60's schematics' parts list is listed as .001 MFD 630VDC B-Box. Which I presume is a film box capacitor. C5 is listed as 820PF 10% 50VDC T/C T&R. Which is a ceramic capacitor. And so on. I'm trying to make an educated guess out of all of this and also assuming whoever engineered this, used those specific capacitors for a reason.

Using a 2200uf cap in C14 supposedly extends the bass freqs down to 150hz from the stock 350hz.

The list of mods I've found online are the following. I've condensed what they supposedly do, for your time's sake. I'm only doing mods 1 & 2. I'm not exactly sure what the other mods are about.

1. Change the coupling caps (c4, c5, c14) out for .0022uf caps. Thickens up the preamp alot and get rid of the cocked wah sound. This will Also warms up the clean channel too.
The coupling cap(C14 .001uF) feeding v2a rolls off everything below 340hz, replacing this with 0.0022uf (aka 2200 pf) makes it roll off at around 150hz, restoring some bottom.

2. The Mid control on the clean channel EQ doesn't really work so well. As you turn it clockwise it adds more mid-bass, but keeps an overall scooped response. This is caused by a capacitor across the Mid potentiometer. Peavey used a .015uF cap (C9) there. You could simply remove it, but that will result in the mid pot turning into more of a gain control considering its 50k ohm value. Instead, I recommend replacing C9 with a smaller value, between .003uF to .005uF.

3. Change the preamp tubes voltage from 260v to 320. Change R33 and R34 out for 4.7k resistors. This will bring this preamp to life (????).

4. Change plate resistors to 100k/200k (R3 and R13 only) This will give less distortion but a rounder gain. The stock resistors are 150k. The highest you should go is 200k, anymore and it will oscillate.

5. Replace the ultra gain pot with a 50k This will make it a clone of the crunch channel

6. The effects send & main outputs beef-up (???). replace C40 & C30 each with a 1uF 400V film cap.

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Enzo
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Re: Rockmaster Preamp: General modding questions

Post by Enzo » Sat Aug 28, 2021 9:50 pm

Yes, every part is chosen for a reason, but rarely a reason you'll know. Now in all the mods you see, I'd wager none of them tell you to replace a 1uf film cap with a 1uf cap of some other type. A lot of choices bear on space on the board, and assembly needs. You are assembling things by hand, but these boards were built on pick and place machines. You can bend a part into place, the machines need straight shots and clearances.
C5 is listed as 820PF 10% 50VDC T/C T&R
You skipped over the T&R, which means tape and reel. The parts come on strips to feed into the assembler machine. If you shop at Mouser you often see the same part with different part numbers, only difference is over to the right. One says bulk, one says tape, etc. COmpanies making stuff need their parts ready to use.

Yes, C14 is a film cap. It has a compact shape and the radial leads are convenient for the P&P. But if you want to change the value, there is nothing requiring you to also use a box film cap. Never lose sight of the fact these are just guitar amps, not calibrated precision laboratory instruments. The changes from your mods will be due to the part value, not its underlying type.

dicianne
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Re: Rockmaster Preamp: General modding questions

Post by dicianne » Sat Aug 28, 2021 11:20 pm

Enzo wrote:
Sat Aug 28, 2021 9:50 pm

Yes, C14 is a film cap. It has a compact shape and the radial leads are convenient for the P&P. But if you want to change the value, there is nothing requiring you to also use a box film cap. Never lose sight of the fact these are just guitar amps, not calibrated precision laboratory instruments. The changes from your mods will be due to the part value, not its underlying type.
Ok good. That's what I wanted to hear. Thanks Enzo!

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The Guitar
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Re: Rockmaster Preamp: General modding questions

Post by The Guitar » Tue Aug 31, 2021 9:47 am

When I had one, I replaced c14 with an orange drop cap. I don't remember the values of it off hand, but that mod eliminated the DOD R31 EQ out of the rack. 😋

*Edit* That cap value has been stated above.

dicianne
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Re: Rockmaster Preamp: General modding questions

Post by dicianne » Mon Sep 06, 2021 9:41 pm

You know what. I didn't really notice at first, but that yellow cap in c5 in the picture I posted above, it's a 2200pf capacitor. So this 100v unit already had this mod installed. I've just realized that other than looking at youtube demos, I don't even know how a stock rockmaster sounds like in person. So both units had more mods than I knew. And both PCBs look like they've been through wars (rust, scratches, lifted traces, flux gunk). I really like how both sound, that I almost want to keep both. I wouldn't discourage modding these units. But it would be nice if people could disclose whether they've been modded or not in their auction/sales posts. I was none the wiser until the first unit failed, and I had to start peaking inside to see what was up. I guess people have been buying and selling these without knowing that. I've got both 120v/100v units working now. Although the 100v version now doesn't output audio on the XLR output due to my clumsy/shoddy work on it that ruined one of the output transformer's lead. I'm not touching these units ever again other than to experiment with different tubes. I've burned out, and I'm happy with the outcome anyway.

I'm done here. Thanks for your guidance Enzo
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