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Marshall: JTM45 vs 1987 circuit. BIG gain difference?

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6/19/2005 2:46 PM

Marshall: JTM45 vs 1987 circuit. BIG gain difference?

Can you really crank a 5F6-A Bassman (AKA old JTM45)to 10 and it just kinda purrs? But where is the CLEAN volume?

You see, I've built a single channel marshall/bassman type amp with a switch that adds a .68 cap across the 820 ohm cathode resistor. So basically without the cap it's a Bassman/JTM45 and with cap engaged it's a 1987/1959 Superlead circuit.

The difference in gain is dramatic to say the least. in JTM45 mode I can turn up the volume all the way up and I get a nice smooth distortion that is not super loud but still sounds good.

In "Superlead" mode it is as loud at 1/4 to 1/3 volume setting as the totally cranked JTM45 mode is. If I crank it to 10 the amp just squeals.

I thought I'd get a louder clean tone in JTM45 mode but the amp doesn't really have any useful volume until at least 1/2 way up. Does that seem right to you?

How could this circuit ever be useful as a bass amp?

I'm seeing 180vdc on the 1st stage plate. If I inject a 1k/1v sinewave I get 41 vac off the 1st coupling cap. Does that seem like the correct gain?

The amp seems plenty loud as a "Superlead" but not loud enough as a "JTM45". Should there be that much difference in gain between the two circuits?

(Cathode on 1st gain stage is 1.5k w/a 15uf cap.)

6/19/2005 3:44 PM

Hi Dave

A 5F6A does have more gain then a JTM45.
There are a couple of differences between a JTM45 and a plexi 1987 (notes on plexi);
- seperated V1 cathodes; different voicing and tighter low-end
- 470k mixing-resistors; slightly tighter low-end and sweeter highs
- brightness- and coupling-caps (tighter low-end, more mids and highs)
- cap on V2 cathode
- tonestack (fatter, less loss in gain)
- gainfactor poweramp (gain in PI)
- output- and powertransformers, chokes
- coupling-caps to outputtubes (tighter low-end)

A JTM45 should be pretty clean till around 5 or 6, plexies with a 5000p brightcap do rock above 2

Try a 820/.68-4u7 on the 1st cathode. What's your neg.feedback-resistor and what iron are you using? What's the B+?

Hope this helps,

Chris
Love, peace & loudness
http://www.CMWamps.com

6/19/2005 7:07 PM

Since the early JTM45's were supposed to be a copy of a 5F6-A I can't see why the 5F6-A would have more gain. I've got schems of those early 45's and they are the same circuit-wise.

I'm using a Delft/Obsolete Electronics JTM45 RS spares repo OT. The power supply has power scaling and sag control with a Twin Reverb PT. At full up I'm getting 460V on the plates. (KT66's,45ma per tube)

B+ on the Input stage plate is 180v some schematics show 150v in this position so I think we're in the ballpark a tad on the high side

I've got a pull pot on the presence control that lets me switch the feedback resistor to 27k or 47k.
There's a subtle gain change but not as significant as the cathode bypass cap switching.

It's like a double "Fat" switch made with an on/off/on switch for the 1st stage cathode.
Up position = 15uf cathode bypass (1st)
Middle position = .68 cathode bypass (1st)
Down position = .68 cathode bypass on 1st
and 2nd gain stage

The down position is the one that really sizzles, kinda like the bright channel of a 1987 or 1959 Marshall. I might try the lower value cathode cap on the 1st stage.

Instead of parallel channels I'm using V1B as a gain stage in series with the input stage. This give the amp lots of grind compared with jumpering the two parallel channels.

Thanks for the feedback Chris; sounds like the circuit (JTM45) is doing what it's supposed to do. I still can't see how this could ever have been a good circuit for bass unless you like to play real quiet.

Db

6/19/2005 7:41 PM
anonymous	If you're running v1b in series instead of parallel, that gives you a lot more gain right there.

6/20/2005 5:02 AM

Hi Dave

"...I can't see why the 5F6-A would have more gain. I've got schems of those early 45's and they are the same circuit-wise."

If you do connect the 27k resistor to a 2 ohms tap you'll get more gain then with the same resistor connected to the 16 ohms tap.

The less voltage is fed to the 27k/presence-pot-junction the less feedback so more gain, the voltage at the 16 ohms tap is higher then at the 2 ohms tap. I can calculate an alternative value but I need more coffee

LOL (but feel free to contact me if needed).

I'd forgot to mention this: the tweed Bassman does use a 12AY7 as V1 and there isn't a cap across one of the mixing-resistors going to V2 so the 5F6A does have less pre-amp-gain but more power-amp-gain.

Try getting that plate-voltage around 150 Volts, the sound will become somewhat warmer and a bit more "gainy".

Vintage bass-amps: imho the 1st real stage-bass-amp was the Ampeg SVT.
Maybe they used smaller bass-amp so you couldn't hear that guy that couldn't play guitar being forced to play bass

6/20/2005 6:46 AM
Chris@CMWamps	and after some coffee Hi Dave For a 5F6A PI/power-amp-gain with a 16 ohms output at 30-50 Watts you can use a 82k neg.feedback-resistor. Hope this helps, Chris Love, peace & loudness http://www.CMWamps.com

6/20/2005 9:46 AM
dave b	Thanks Chris I'LL check what tap I'm getting feedback from. Db

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