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FDP Forum / Amp Mods, Repairs, and Projects / DRRI master vol mod?

bongo122819
Contributing Member

usa

All I need is one more guitar
Nov 22nd, 2016 01:00 PM   Edit   Profile   Print Topic   Search Topic

I currently use my DRRI with a Weber Mass Attenuator (50) and like the ability to get the sweet cranked sound at reasonable volume.
It's a bit of a hassle to carry to a job and hook up, etc.
A local tech said he could add a Master volume control to get the same effect but I'm confused.....thought the Attenuator drives the power amp section (hence the nice distortion) but a Master volume drives the preamp?
Can somebody clarify? thanks

NHILL1

United States

Nov 22nd, 2016 01:47 PM   Edit   Profile   Print Topic   Search Topic

I've asked my tech a similar question, he said one way is to adjust the negative feedback loop to add more gain and less headroom. The other way is to overdrive the pre amp and cut the signal before it gets the the power amp.

Also, I used a weber attenuator before and I hated that thing. I'd just get a blues driver, sounds real fendery and is just a real complimentary pedal for a fender.

Another option, get a blues jr. It has a master volume, lots of gain and is very portable. I had thought about doing the same thing to my DRRI, but I just let it be and got a blues jr. It does the fender overdrive sound exceptionally well, but...

Here's an example of a SRRI with a master volume installed, I must say I'm impressed with that sound. Maybe ask your tech if that's something your amp will be able to do.

Master Volume

(This message was last edited by NHILL1 at 06:56 PM, Nov 22nd, 2016)

Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
Nov 23rd, 2016 06:20 AM   Edit   Profile   Print Topic   Search Topic

A post PI master would work and work well. The PI is part of the power amp. Much of what people think "power tube distortion" is, is the PI driven to breakup and the power tubes simply amplifying that.

I've put dozens of post PI masters in tube amps. I usually use a dual, stacked pot, 1meg audio right after the PI coupling caps (wired as volume controls) and add additional blocking caps after the master to block the bias voltage. The new blocking caps only need be 100v power handling. Use 10 times the value of the PI blocking caps to maintain bass at lower levels.

A simple crossline master works fine in a 6V6 amp. I use a 250k audio NO LOAD pot. Some like a 100k audio no load pot. A 1meg audio pot will work but isn't as smooth.

In my DR I moved the intensity pot for the vibrato to the ext speaker jack hole and put the master in the intensity hole.

Hammond101
Contributing Member
**********

So. Cal. USA

Nov 23rd, 2016 09:51 AM   Edit   Profile   Print Topic   Search Topic

The post PI method works great actually. Easier to fashion in a hand wired DR but it could be done with the DRRI.

I blackfaced my '75 TR and kept the master volume. Preamp warmth with output attenuation, heaven! With both the volume and master set at 5 the thing is just awesome.

Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
Nov 28th, 2016 03:55 PM   Edit   Profile   Print Topic   Search Topic

With a post PI master, there is enough gain in the preamp stages to break up the PI and get decent and abundant distortion with very good tone. I've done so many Fenders (and Marshalls) and ever Fender had a slightly different voice that comes out with the post PI master. It was always fun to hear each individual voice.

And not a clunker among them.

Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
Nov 28th, 2016 03:56 PM   Edit   Profile   Print Topic   Search Topic

Fender DID make an amp with a post PI master...the Zinky designed Tone Master. A monster of an amp.

avspecialist

USA/Connecticut

Bobby Roadrunner
Feb 12th, 2017 07:00 AM   Edit   Profile   Print Topic   Search Topic

Hi Steve: I hope all has been well with you. is there a diagram available on how you wire in the post master vc you are talking about? I have looked at the train wreck papers and others, but everyone's layout is different. I know with your experience your layout would be best, thanks, Bob

pdf64

UK

Feb 12th, 2017 09:40 AM   Edit   Profile   Print Topic   Search Topic

It may be helpful to identify the exact amp you are looking to mod, and which type of master vol you wish to install.

The regular pot required for a type 3 crossline is easier to source than the dual track pot needed for the type 1 or 2.

My experience is that the type 3 crossline tends not to sound good with a 12AT7 LTP.

avspecialist

USA/Connecticut

Bobby Roadrunner
Feb 12th, 2017 03:20 PM   Edit   Profile   Print Topic   Search Topic

Hi pete, i'm playing around with a 1974 bassman 50 that i have done half aa864 and half ab763. it had a single pot master in it, which i didn't like. i've done a dual gang post mv, but i did not use 2 sets of caps as described by Steve. Usually i like to work off a layout so i can make sure i'm installing right.

pdf64

UK

Feb 13th, 2017 02:55 AM   Edit   Profile   Print Topic   Search Topic

The MV diagrams in the Trainwreck pages are a layout of sorts; the Type 1 seems to be what Steve is referring to.
After working in that area of an amp, it's a good idea to do an initial power up without the power tubes in place, in order to check that all the voltages, especially to the control grid pin 5, are still ok.
robrob has some nicely drawn layouts on his site, though I don't know if he's drawn a type 1 MV.

External link

(This message was last edited by pdf64 at 04:57 AM, Feb 13th, 2017)

Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
Feb 13th, 2017 06:20 AM   Edit   Profile   Print Topic   Search Topic

I use what the Trainwreck pages calls #1. The extra two blocking caps though, should be 10 times the stock blocking cap values. Since these caps only block the bias voltage from the pot, they need not be that high in voltage handling. 63V or higher is fine. I use 1uF/100v poly or metal caps.

I prefer the stacked pot masters in larger amps, but a cross line master works fine in smaller amps. I use a 100k-250k, no load pot for cross line masters. I make my own no-load pots.

Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
Feb 13th, 2017 06:20 AM   Edit   Profile   Print Topic   Search Topic

I use what the Trainwreck pages calls #1. The extra two blocking caps though, should be 10 times the stock blocking cap values. Since these caps only block the bias voltage from the pot, they need not be that high in voltage handling. 63V or higher is fine. I use 1uF/100v poly or metal caps.

I prefer the stacked pot masters in larger amps, but a cross line master works fine in smaller amps. I use a 100k-250k, no load pot for cross line masters. I make my own no-load pots.

avspecialist

USA/Connecticut

Bobby Roadrunner
Feb 13th, 2017 02:56 PM   Edit   Profile   Print Topic   Search Topic

Thank you Pete and Steve, I will look at again, thanks !!!

avspecialist

USA/Connecticut

Bobby Roadrunner
Feb 13th, 2017 03:03 PM   Edit   Profile   Print Topic   Search Topic

Looking at the Trainwerck type 1, is different than what a lot of MV kits say. they remove the 2. 220 resistors and put 2.2 meg on the pot.. what's the difference?

pdf64

UK

Feb 14th, 2017 02:31 AM   Edit   Profile   Print Topic   Search Topic

The normal 'LarMar' arrangement that's shown on a lot of internet sites is a version of the the trainwreck type 2.
That puts the dc bias voltage on to the pot tracks, which some pots may not be rated for.
If you read robrob additional info about master volumes (which I linked to previously) you will be able to see something about the rationale for the LarMar changes to the type 2.

Steve, I think that the voltage rating for the blocking caps between a Type 2 pot wipers and power tube control grids should be a lot higher than the bias voltage itself.
This is because the instantaneous voltage across the cap is comprised of the bias Vdc and the signal Vac.
And when the power tubes are overdriven, clipping of the signal at the power tube grids will cause bias shift, which will temporarily increase the effective bias voltage at the power tube grids, even perhaps past cut off. This is known as 'blocking distortion'.
I think that it's important not to risk an overvoltage on those caps, as if they get leaky or fail, the power tubes will lose bias.
To mitigate for bias shift / blocking distortion, coupling cap values are best kept as low as feasible for the required bandwidth.
See link for a bias excursion calculator.
Also Aiken discussed blocking distortion at http://www.aikenamps.com/index.php/what-is-blocking-distortion

External link

(This message was last edited by pdf64 at 05:30 PM, Feb 14th, 2017)

Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
Feb 18th, 2017 07:49 AM   Edit   Profile   Print Topic   Search Topic

I use 100vdc caps and have never had a failure. I use high quality caps. I have built many amps and have modded and repaired hundreds. I know well what blocking distortion is, but have never driven power tubes into cutoff. I've experienced many amps, usually customer modded that drove preamp stages driven into cutoff, due to excessive gain and insufficient padding.

I can't imagine a scenario where the bias voltage would double in a fixed bias scheme and exceed 100 volts during use. I did read the article.



Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
Feb 18th, 2017 08:01 AM   Edit   Profile   Print Topic   Search Topic

Over the years I used #2 only once, in a Bandmaster head for a harp player. While it sounded excellent, I read of some failures of the pot in articles years later. Failure would remove the bias voltage from the tube, causing red-plating and likely tube failure. Many add parallel resistors to maintain some bias voltage if the pot fails.

Soundwise, it was extremely similar to type 1, which I prefer. I have used 500k stacked audio pots (initially) with identical added caps to the stock coupling caps, and later used 1meg stacked audio pots with 10 times the value and 100v...the higher value helps maintain low end at low master settings. I noticed no appreciable changes with the master turned up. In channel switching amps I've made, the master is switched out in clean, and comparing is easy.

The caps in type 2 are much higher handling than the bias voltage, as they are the stock coupling caps after the PI. In type 1, there is an added pair, putting the master between the stock coupling caps and the bias supply.

ejm

usa

Feb 18th, 2017 09:32 AM   Edit   Profile   Print Topic   Search Topic

There are several ways to add a master to an amp. Discussions and exchanges of information like this are good to know and file away for possible future use, as well as one's own education.

However, if you like the sound of the amp with the attenuator, I'd stick with that and leave the amp unmodded.

Even though the amp isn't vintage, you'll still potentially lower the resale value. And even though you may *think* that you'll never sell it, you never know.

You are correct about the effect on the amp. A power attenuator lets you drive everything but the speaker and keep the volume low. The PPIMV lets you drive all of the preamp tubes but not the power tubes. Of course, there are some that will debate whether or not you ever really drive the power tubes to the point of their having an effect unless you REALLY wind the amp up, but that's another discussion.


Steve Dallman
Contributing Member
**********

Merrill, Wisconsin

Age is just a number...mine is big
Mar 13th, 2017 12:22 PM   Edit   Profile   Print Topic   Search Topic

The PI IS part of the power amp. Much/most of what we call "Power Tube Distortion" is the PI breaking up, and that breakup being amplified by the power tubes. Even if the power tubes get to breakup, it is often masked by the amount of PI distortion.

A post PI master allows this important part of the power amp....the PI to be overdriven. It sounds good.

FDP Forum / Amp Mods, Repairs, and Projects / DRRI master vol mod?




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