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which is the higher gain between the trinity plexi and trinity TMB in your opinion ZP?

geoff 1965 wrote: ↑

Thu 05/13/21 7:06 pm

which is the higher gain between the trinity plexi and trinity TMB in your opinion ZP?

EditedI':It's been a few years since I was working with Trinity, which was before I had the surgery and radiation treatment on my brain. However, my recollection is that those two TMB variants from Trinity have around the same level of preamp gain. The "Plexi" 18W variant just has Bright and Normal channels in its TMB preamp, like a regular big Marshall Plexi (1959). It also has a Plexi-style 470 ohm cathode resistor in the PI, instead of the 820 ohm one of a regular 18W.
This gives increased gain in the PI, as well as a thicker PI distortion quality.
Hope that explanation helped!

Paintwork done,time to install the transformers & tube sockets “then” the dreaded heater filaments!
note;just redoing the front lettering,did'nt notice the slight slant until i posted pics!

Transformers & filaments wired in,vvr unit and switchable rectifier next.
I contacted trinity amps about the gain level of their 18W amps and had an awesome response from them,i’m leaning towards the plexi preamp,don’t want too much overdrive with this amp.
I’m going to bias the pentodes the same as el84’s at 180R and i’ve got 50,100,470 & 1000uf capacitors to experiment with.

here's the schematic for the plexi, with "slight mods!" i think this will be good.remove R8 and i won't be using the clean boost so i'll try 2K7 or 1K5 on that cathode and see which is best.not sure about the screen 100R's either with the ecl's will probably use a 1K5 dropper then 1K's.
6 X 22nf couplers in this amp,7 with the tube buffer!

geoff 1965 wrote: ↑

Tue 05/18/21 4:33 pm

18WattPlexiSchematic9.pdf
here's the schematic for the plexi, with "slight mods!" i think this will be good.remove R8 and i won't be using the clean boost so i'll try 2K7 or 1K5 on that cathode and see which is best.not sure about the screen 100R's either with the ecl's will probably use a 1K5 dropper then 1K's.
6 X 22nf couplers in this amp,7 with the tube buffer!

I really don't at all like that unbypassed 2.7k cathode resistor on the 2nd preamp stage, which both limits gain and reduces the high-end detail (with or without the clean boost switch) . In a real Marshall 1959 Plexi, that resistor is 820 ohms(non-bypassed). I don't know why it wasn't left at that well-tried and proven value in the 18W Plexi.

Yeah the notes for tweaking the gain show 470R to 2K7,i’ll try all of them ZP and see which gives the best tone/gain.that’s one good thing with the tag strips it’s going to be easy swapping components in and out.
What’s your opinion on the 470R in the phase inverter with the ecl pentodes, i.e. will they respond the same as el84's?

geoff 1965 wrote: ↑

Wed 05/19/21 2:42 pm

Yeah the notes for tweaking the gain show 470R to 2K7,i’ll try all of them ZP and see which gives the best tone/gain.that’s one good thing with the tag strips it’s going to be easy swapping components in and out.
What’s your opinion on the 470R in the phase inverter?

If you want the pre to overdrive more easily, use Marshall's value of 820 ohms at the cathode, but raise the value of R7 to 180K or 220K. Changing R7 won't affect gain too much (a couple dB), but the second section of the cascade will clip much sooner. It will be asymmetrical, so plenty of even-order energy. Just something to try if you get bored.

Jack

Thinking of trying the same on V1 as well Jack,220K plate & 820R cathode.

Won't have the same effect at V1. It's the direct coupled cathode follower that clips when the other anode resistor is increased.

Jack

geoff 1965 wrote: ↑

Wed 05/19/21 2:42 pm

Yeah the notes for tweaking the gain show 470R to 2K7,
What’s your opinion on the 470R in the phase inverter with the ecl pentodes, i.e. will they respond the same as el84's?

Yes, the effect will be the same with regular EL84s or the ECL86 (6GW8) pentodes

The 470R will give you a nice bit of overdrive within the PI, while the power tube overdrive remains the same.

Vvr & switching wired in,now careful planning of component layout and lead dress.going to put the tonestack parts between the power & phase inverter so I can jump the couplers directly to the pots from the tags.

Power section grounding done,cathode wiring done i’ve biased the pentodes at 180R/100uf to start with,had good results with that combination in my lite2.just waiting for some resistors to build in the phase inverter then I can test the vvr and see what plate voltages i’m getting.loving this! tricky part is double checking the pin outs of the ecl’s when for the last 4 years it’s been el84’s & ecc83’s implanted in my brain!

Has anyone got any reference plate voltages for V1 & V2 in the plexi circuit? my preamp B+ is direct “no vvr” so I need to adjust that to suit,trinity has 272V on pin6 of V2 and i’m wondering if that will be okay or is there a “halcyon” voltage to target. some good progress today!

Nice work!

From the input Jack to pin2 of V1 is only 2 inches,so do I make a very short shielded cable or risk jumping the grid resistor across?
I found some Marshall plexi reference voltages to compare with the trinity,quite a bit of difference in plate voltages due to the cathode biasing I.e. the trinity has 820R/.68uf and the Marshall 2K7/.68uf I imagine there will be quite a difference in tone as well,will experiment with that one.already replaced the clean boost values with 820R on the V2 cathode.

geoff 1965 wrote: ↑

Wed 05/26/21 6:15 pm

or risk jumping the grid resistor across?

How is that a risk? Do you not trust your soldering skills?

risk as in the resistor leads acting like mini antenna's Josh,or am i being overly concerned there?

geoff 1965 wrote: ↑

Wed 05/26/21 8:13 pm

risk as in the resistor leads acting like mini antenna's Josh,or am i being overly concerned there?

Overly concerned. If they act as antennas, then consider doing something else. But 2 inches isn't likely long enough to hurt anything.

Okay i’ll try that,can do the same with the gain pot as well.