OT: Tube amp questions

[Arno van der Heijden]

I recently bought an old 50 watt Roost head. It's the same as this one: http://www.renton-world.com/roost/guts.html
But without the reverb and with 2x EL34 and 4x ECC83

The schematic should deviate not that much from this one (I know it has some mistakes):
http://www.renton-world.com/roost/roost.pdf

The problem is that I don't like the master volume very much. When it's dimed, the sound gets very dark and bassy. I have to turn it up to 8 (very loud) to get a decent sound. Would help to change it to a post PI master volume?
I don't have any experience with these dual channel tube amps, but I was wondering, using a post PI MV, if it is possible to set one channel for a clean sound and the other for high gain sounds, so I can use an a/b switch to switch between channels? Wouldn't this give me to much differences in volumes between the channels?

I was thinking about some other mods. Does anybody have some ideas for that?
If that schematic is right, there's an unused triode I could use for an extra gain stage...

Also, I was wondering what's the advantage of this kind of PI? It needs an extra triode.
If I was to add a power amp input jack, where would I insert it?

Any advice is welcome...

[jplaudio]

I recently bought an old 50 watt Roost head. It's the same as this one: http://www.renton-world.com/roost/guts.html
But without the reverb and with 2x EL34 and 4x ECC83

The schematic should deviate not that much from this one (I know it has some mistakes):
http://www.renton-world.com/roost/roost.pdf
Looks like a HIwatt copy for the most part
The problem is that I don't like the master volume very much. When it's dimed, the sound gets very dark and bassy. I have to turn it up to 8 (very loud) to get a decent sound. Would help to change it to a post PI master volume?
A post PI won't improve the voicing. You could try rolling off the low end earlier in the circuit.
I don't have any experience with these dual channel tube amps, but I was wondering, using a post PI MV, if it is possible to set one channel for a clean sound and the other for high gain sounds, so I can use an a/b switch to switch between channels? Wouldn't this give me to much differences in volumes between the channels?
This is not really a dual channel amp as much as it is a dual input design. Both"channels" are mixed after the first stage and share a common tone stack. A post PI MV will not allow switching between clean and high gain using the dual inputs. You would need to design a way to allow independant master and gain levels for two channels plus a way to switch between them.

I was thinking about some other mods. Does anybody have some ideas for that?
The basic components are there do some cool stuff. I would redo the entire preamp section to tighten up the low end and provide additional gain plus maybe provide two separate channels for clean and OD. The only problem with a two channel design is having enough holes in the front panel to accomodate the extra controls you may need. Another way to mod it would be to use a switched gain stage design where you alter the gain structure rather than switch between two independant preamps, kinda of like a dumble ODS. You might want to look at JKB's Transatlantic design for some ideas
If that schematic is right, there's an unused triode I could use for an extra gain stage...

Also, I was wondering what's the advantage of this kind of PI? It needs an extra triode.
The PI is a LTP type. The extra stage is a cathode follower  at the input of the power amp. This probably provides a very clean drive signal for the LTP and allows the NFB to be input before the PI. Also the position of the presence control allows it to boost/cut the high end.
If I was to add a power amp input jack, where would I insert it?
The power amp input is the grid of V4b

Any advice is welcome...

[Nasse]

Looks impressive amp, that would be dream to modify and customize, or just restore...

I don´t have much to say about tube amps (I like mostly solid state for bedroom playing) but was thinkin something about your story about that master volume muddiness problem. Of course I hope you don´t get zapped with high voltages.

I was just cleaning my archives, searched for some "lost" stuff that I had archived too "well". Somebody asked something about loudness control in old home stereo amp, and I had just found one old schemo about such... And Mark Hammer (?) mentioned something that treble bleed and bright circuits are somewhat similar... So what´s the idea?

Treble/bright cap is not effective at higher volume settings, and "standard" loudness circuits are done with hard to find or obsolete potentiometers with extra tab. I found two circuits in different magazines, both suggested same method to solve difficult setting and special pot problem for loudness circuit: Instead of complicated and less than satisfactory working one-knob loudness circuit build it with two pots, a mixing stage and one control for straight signal an another for "loudness". With such arrangement you can dial perfect match at any volume.

Loudness circuit is basically a passive mid scoop circuit (you want more highs and lows at low level to make it sound "louder" or more natural, but it is easier just scoop mids down and raise overall level)

[puretube]

try: 220p from wiper "master" to wiper "treble" pot...

[Arno van der Heijden]

Thanks for the comments everybody!!
I actually haven't tinkered with tube amps before, but I have a basic understanding of the different 'building blocks' they are made of. And yes, I know there are lethal voltages involved.
Also, it should be noted that this amp is build on pcb, which makes it more difficult to mod.

Looks like a HIwatt copy for the most part

Which model(s)?

A post PI won't improve the voicing. You could try rolling off the low end earlier in the circuit.

I meant to say that the amp sounds good when the MV is above 8. My reasoning was that this partly comes from the fair amount of preamp gain that is created AFTER the master volume.
My guess is that if I make the MV post PI, I get a better sound with the MV dimed, because the amount of preamp gain is left unaffected.

The basic components are there do some cool stuff. I would redo the entire preamp section to tighten up the low end and provide additional gain plus maybe provide two separate channels for clean and OD. The only problem with a two channel design is having enough holes in the front panel to accomodate the extra controls you may need. Another way to mod it would be to use a switched gain stage design where you alter the gain structure rather than switch between two independant preamps, kinda of like a dumble ODS. You might want to look at JKB's Transatlantic design for some ideas

How do I tighten up the low end? Would a two channel design neccesarily need a second tonestack?
Where can I find JKB's Transatlantic design?

The power amp input is the grid of V4b

Do I need to add an input cap? If so, what value?

I was also thinking of adding a switch to switch between pentode and triode operation. Would that be a useful addition?

[WorkBench]

Check out www.HoffmanAmps.com  The forum there is great for tube amp talk.  I built my twin with the help from those guys.  It is amazing!
Chris

[Lonestarjohnny]

You can tighten up the bottom end by changeing the 2 x 200uf caps  on your B+ out for 300uf x 250vdc cap's, I would use a nice set of Atom's,
You could do a Split Phase inverter M/V, you would need a dualstacked 500 k pot, and 4 ea. of .022 by 600v.orange drops, remove the 2 .047 cap's from V3 that feed your power tube's, and replace each .047 cap with 2 .022 cap's in serie's, tie 1 half of your dual stacked M/Vpot right in the center of the .022 cap's, I've used this on some of my amp's and I like it very much, I saw this mod in a Trainwreck amp I got to work on once. also I would run Shielded wire and only tie the shield on 1 end to grd. at the pot.
JD