New project: Super Bass + Hot Rod 2203 in one amp.

[Dom]

Hi folks,

I'm planning to build my own 'signature amp' that combines the full roar of a Super Bass with the sting of the Caswell/Levi mod (I'm not aiming for a 100% AFD sound. It has to suit my ears and hands, not Slash's).

Transformers: PT Marstran Dagnall T2562, choke Marstran C1999, OT Metroamp C1998

Filtering
Mains 50 (2x 100uF)
Screens 33uF (4x Rifa 33uF)
PI 33uF
CF 33uF
Superbass 'V1' 22uF
2203 'V1' 33uF
#36/#39 stage 22uF

This amp has 5 preamp tubes, because I want to keep the shared cathode thing, I like that.
Well, the plan behind this preamp is that I can use the hot rodded 2203 stage and the Plexi stage at once, when linked together.
The normal channel of the Super Bass preamp is 'buried' (so the grid goes with a 33K to ground, like unplugged).
the volume knob of the normal channel is on the board and is for fine tuning the amount of bass.
In place of the normal channel there's the gain knob for the modded 2203 section.
The mixer resistors all go to a switch where I can chose the Plexi channel only, Plexi + Hot Rod 2203, or only 2203.
Another feature is a switchable master volume (which is on top of the chassis for shortest possible wire lenghts and tro keep the 6 knob 4-holer look of the amp.

For detail just look at the layout.

I want to introduce the Larry grounding scheme, but I dunno how it will work with 2 different stages in parallel. maybe somebody can jump in and give some advise.
Also if the high tension rail of the preamp is done right for implementing the Larry grounding.
Thanks for any advise in advance!

Cheers
Dom

[Dom]

Here's the schematic of the preamp section to get a better understanding of the whole thing.
The circled numbers are my suggest ground points in regards to Larry-grounding. But I'm not quite sure.

[neikeel]

Hi Dom can you relabel your ECC83 nomenclature? I get the set up for the channel 1 set up (original V1a and V1b as shared cathode) with V2a second stage and then V2b cathode follower.
It is the channel 2 arrangement, presumably two extra tubes but only using half of one, er V -1b into V0a and then V0b and then to cathode follower?
That way you are keeping the 4-stager in circuit all the time on that channel just varying the gain levels?
Do you think that there will be a siginificant difference between 22uF and 33uF in the different modes? 22uF vs 50uF sure but I doubt the others. Personally I would simplify it and use 32 or 33uF fixed

[Dom]

Ok, so the preamp layout is like this ( according to the layout, left to right)
V1a N/A
V1b 3rd gain stage of modded 2203
V2a 2nd gain stage of 2203
V2b 1st gain stage of 2203
V3a 1st gain stage 'normal' channel
V3b 1st gain stage 'high treble' channel

V4 = Plexi / 2203 V2
V5 = Plexi / 2203 V3

So the 2203 channel is always running with the added hot rod stage on.
If it was switchable, then you get phase cancellations, when using both the plexi channel and 2203 channel linked together (the signal is inverted after every single stage)

On the scheme the numbers in circles are the grounding spots (but I'm not sure if this is the perfect grounding).

The filter caps are: V1b 22uF (Roe and Brian W. recommend extra filtering of additional stages)
V2 33uF
V3 22uF
V4 33uF
V5 33uF

[neikeel]

I tried the extra cap and extra droppers for my extra stage but found it gave the added stage too much fizz and lacked punch - whether it was the extra decoupling resistor or the cap - or both! Not sure.

- ooks like a few of us trying the switchable hotrod. I need to get my other projects out of the way first. My blockend JTM50 is nearly there (your chassis). Funnily enough I thought that I was making an amp that never was (Black Flag, block end JTM) but then I go and find one:
http://vintageamps.com/plexiboard/viewt ... ag#p988855

Sorry strayed off topic. Please keep us posted on how it works

[Dom]

nice odd ball 50Watter!
regarding the filtering, that's one thing I have to try out and see what sounds feels better.
I'm only curious if the other decoupling resistors are positioned correct chematic-wise, since there are 2 preamps in parallel.
And I'm also curious how it will sound with the channels jumpered since that's my goal having a nice blend of both a scorching 80s modded 2203 sound as also that Plexi vibe.
Power stage wise it's more Plexi than 2203, I like the lower filtering better.

[Dom]

In the attachment there's a simple concept of my preamp power supply. Would this work fine or are there any concerns about it?

Neil, how did you hook up your filter cap and the voltage dropping resistor? To V1 plates supply or to V2 plates supply?

[neikeel]

In the attachment there's a simple concept of my preamp power supply. Would this work fine or are there any concerns about it?

Neil, how did you hook up your filter cap and the voltage dropping resistor? To V1 plates supply or to V2 plates supply?

I ran a feed from the V1 plates node to a 10k and then to my 22uF can. Funnily enough when I did the 39 mod on an old 67 PA it was sweeter. On that I did not decouple with a 10k simply hooked up an extra 8uF can. That ran 0.1uF couplers which I do not usually like in a lead amp but it worked in this application.

A friend came over this w/e and gave the SIR 39 some stick. We did a bit of EQ ing and actually got it to sound pretty good in 4-stage mode through some T1511s and T1221s in an old b/w cab. Still very bright in 2203 mode and lacks a bit in plexi mode - my 1987/2204 really smokes it in plexi mode (as of course does the '68SL). Too much of a one trick pony at the moment so when I get time I plan a major rework to set it up so the plexi channel works better on its own.

[Dom]

Neil, I assume when adding another 10K R between V1 filter and V0 filter cap that the fizz and lack of punch was due to browning out this stage? Maybe because of the added 10K voltages on this tube were too low and killed headroom for punch.

On the other amp, when you add without dropping R between V0 and V1 a filter cap then it's just a parallel configuration.
In case of a '67 it's 32uF (from the one side of dual 32+32. If it'S a 32+32) + 8uF = 40uF for both V1 and V0.

Well anyway, I went on with my build. A friend of mine asked me, if I can also build another one for him, with different specs, more based on a KT66 loaded JTM45/100 in contrast to mine, which has the Dagnall style OT.
His amp has Sozos and mine has Pihers and Mustards.

On the picture the boards on the top is the JTM45/100 - 2203 Hot Rod and below it's the early 68 Super Bass - 2203 Hot Rod.

[Dom]

Here's a quick fit of the boards into the chassis. This is aluminium (for the JTM45/100 based one). The other with the '68 specs will have stainless steel.

[axeman]

I tried the extra cap and extra droppers for my extra stage but found it gave the added stage too much fizz and lacked punch - whether it was the extra decoupling resistor or the cap - or both! Not sure.

- ooks like a few of us trying the switchable hotrod. I need to get my other projects out of the way first. My blockend JTM50 is nearly there (your chassis). Funnily enough I thought that I was making an amp that never was (Black Flag, block end JTM) but then I go and find one:
http://vintageamps.com/plexiboard/viewt ... ag#p988855

Sorry strayed off topic. Please keep us posted on how it works

Sweet, I'm also getting ready to build a JTM50. Very nice amp

[Dom]

Ok, I finished one of the 2 amps last week. It's the one with KT66 tubes and Wallace's Drake OT.
I did forming the filter caps as Larry suggests and the voltage drop across the 100K was 5.5V after 20h.
It has 2 old Eries 100uF for the mains which I had formed some weeks ago seperately.
I think that's pretty ok for those old caps.

I can post a voltage chart later but the plexi preamp part is in the ballpark of the Dickinson / Hendrix amp.
The 2203 hot rod channel is pretty high with around 240V-285V on the plates of the 3 stages.
Maybe I try a higher dropping R, but I kinda like the dynamics and headroom for this channel.

The amp now has 5h and the sound smoothed out a lot and holds together better than in the first few minutes.
It's known that an amp needs at least 20h before judging the sound.
I compared it side by side with my JTM45/100 clone and the feel and sound is very similar

First I tried the hot rod channel without any cap across 2nd and 3rd stage 10K cathode Rs.
But during rehearsal it didn't cut through the mix so well, so I added a 0.1UF on the 2nd stage's 10K and this shifted the frequencies in a more usable direction without getting added mush. The distortion is defined and clear. Maybe I'm looking for more twang on the lower strings.

The JTM45 channel is very quiet, but the hot rod channel hisses a lot.
Better trying a 500K as gain pot like Roe suggests?
I currently have a 1M with a 150K from middle lug to ground.

Presence squeals also above 7, but I try today with a proper shielding. I only had the open chassis with the perforated metal plate of another amp's back.

[neikeel]

Looks good and sounds like you are on the right track.

I would be interested in seeing closer pics of your preamp layout when you have settled on a design.

I need to find some time for this stuff but work and family time is more pressing at the moment.

[draytonmodder]

Can I ask why you're using old caps on the new build? I have never understood why people do this. You spend time "forming" them and you get a cap that I perosnally wouldn't trust if I want to crank it up.

There's a lot of hype around caps, but electrolytics aren't a place to try to go vintage, especially when companies like F+T are making great caps that are smaller and do the job better. Even for cap cans, F+T or JJ's get the job done and are a lot more reliable, especially if you want to push the amp any.

I have done similar builds to this and keep the power supply solid with new production caps. You will appreciate the low end performance and reliablility. Also, if you want to have 2 very different channels, run 2 different tone stacks or you will drive yourself nuts trying to voice the channels. The JCM and Plexi tones really need to be EQ'd seperately if you want the best of both.

[Dom]

I use the old ones, because they're cheaper. I wanted to use Sozos, but he didn't had those '67 caps on stock.
Better?
what is better?
Small size doesn't matter, my experience the bigger the cap the better the tone (not always, but most of the time).
I have F&Ts in another amp fo mine, I don't like them in a JTM45 i.e.
So I went for Rifas and well, as said before, the Eries were cheap and they formed well.
The map has now about 60 hous, played it full tilt for most of the time (with a silencer).
No problems so far.
and I have no problems to run the channels. Doesn'tmatter seperately or blended together.
It's JTM45 typicall for the Ch1 (B 0-2, M 7-10, T5-10) and CH2 is very versatile.
I like it and see no reason to change it.