Bedroom Plexi PPIMV

[Littlewyan]

I think i'll buy a hammond chassis for my amp, that way I can get a bottom plate which will act as a shield. Good way of making a chassis though, thats what Marshall do for some of their amps.

I'll probably use the JTM1 circuit I posted earlier, although I am a bit nervous about its validity. Also unsure about biasing the 12AU7 in PP so need to do a bit more reading. Also just found that Marshall released ANOTHER JTM1 called the JTM145 but I don't think we'll see a schematic for that for a while. Almost tempted to buy one but £600!!!!!!

[galtjunk]

Take a look at the thrifty plexi over at watkins.com in the Tone Per Buck section..
It uses cheap russian tubes and puts out about 4 watts in pp.
The 6n2p is a 6 volt 12ax7 and the 6f4p is a 9 pin ecl84.
The tubes only cost a couple of bucks each.
The 6f4p's like 14 to 16k, but if you put a 16 ohm load on the 8 ohm tap it will get you close.

MPS will wind a 14k, 16k OT for a reasonable price. I think it cost $36.

Pure Jimmy Page tone.

You can add a decade switch like a Fargen Miniplex by switching out the cathode cap on the first 6n2p from a .68 (60's) to a .22 (80's) or switch it completely out (70's).

[Littlewyan]

Ah that would require buying new valves. I'm trying to use up bits that I have spare. I have 3 x 12AX7s to spare and 2 x 6V6s. 6v6 Plexi would still be too loud though so I thought if I built a JTM1 I've only got to buy a 12AU7.

[Littlewyan]

I've been reading up on biasing the 12AU7 in Push Pull and done some experimenting in LTSpice. Now the 12AU7 at 250V on the Anode needs -9.5V on the grid or 9.5V on the Cathode. The JTM1 uses Cathode Biasing so we'll go with 9.5V on the Cathode. They use a 470ohm resistor for the bias and in LTSpice its spot on at 9.5V. At 300V the Cathode should be at 11.5V and if I increase the Anode Voltage to 300V the Cathode goes up to 11.5V. So does the Cathode Resistor not need to change or is this just LTSpice not being accurate?

[printer2]

I got a recording of a work buddy trying my amp. I have a link to a download site if you care to take a listen.

https://tubeamparchive.com/viewtopic.php?t=25023

[jazbo8]

if I increase the Anode Voltage to 300V the Cathode goes up to 11.5V. So does the Cathode Resistor not need to change

Cathode bias is self regulating, so you can just leave the resistor as is, although, the bias seems very hot...

[Littlewyan]

I worked out that its at about 70% dissipation. The 470ohm resistor is shared by both sides of the 12AU7 so the triodes actually see just under 1K. I'll measure the bias once the amp is built to make sure.

[Littlewyan]

I just want to ensure that I'm working out the bias of the 12AU7 correctly. I've used Merlin's website as a guide and biased the 12AU7 going by the graph on the datasheet. Now some of the lines on the graph I've had to estimate as the current on the graph doesn't go high enough! It seems that the JTM1 with its 250V Anode Voltage and shared 470ohm resistor is biased at 100% dissipation! I've drawn my lines using a 22K Load though as that is the transformer I am using, Marshall used a 18K Transformer, however that just biases it even hotter! Am I doing something wrong here?

I drew the first line by doing 250 / 11000 (half of the A-A Impedance) = .022A. Then carried on drawing lines up until I got to the max dissipation curve. I couldn't do 250 / 5500 (Class B Line) as the graph didn't go high enough with the current.

[jazbo8]

Am I doing something wrong here?

I drew the first line by doing 250 / 11000 (half of the A-A Impedance) = .022A. Then carried on drawing lines up until I got to the max dissipation curve. I couldn't do 250 / 5500 (Class B Line) as the graph didn't go high enough with the current.

Yup, that's way Marshall are designed! Nothing wrong with what you are doing, Marshall runs the the tubes hard/hot, look at some of the best known ones such as the 2203, and you can see that the output tubes are operated way over the "Design Center" values - that's a part of the magic.

[Littlewyan]

I think I do recall actually that Ken Fischer told Glen Kuykendall (he can confirm this) that as long as the valve doesn't red plate then you can bias it as hot as you like. I'll stick with Marshall's value for now though and perhaps experiment once the amp is built.

Once I've built it I'm going to experiment with a boost switch. Going to try adding a cathode bypass cap onto the second stage on a switch. I'm open to trying things but I suspect that this might just overdrive the power valves too much. So maybe a switchable NFB would be better? I know the JCM1 has a lot more gain yet it has the same number of gain stages bar a MOSFET Source Follower for the Tone Stack. However I don't think the Source Follower will enable the amp to have that much more gain. James M did say the NFB is SIMILAR, so I'm thinking its reduced?

What do you guys think?

[jazbo8]

Don't know how I missed this thread, but it has a lot of good information and soundclips on the JTM145/JTM1. Even at 0.1W, the clips sound great - I am actually surprised a bit, I mean the speaker cone hardly even moves, right?

As for the boost/Fat switch I think it is a good idea to have, just an extra hole to drill and a few more parts, why not...

[Littlewyan]

Yer the speaker is getting a nice break when its in .1W Mode, I mean in that mode it actually runs at 25mW so the speaker is doing sod all! Read through that thread, read through the Brazilian thread using Google Translate and read through a long thread at EL34 World. No official Marshall schematic though. I'll have to experiment with the boost switch once its built as like I said, I could add a bypass cap to one of the 2 remaining unbypassed cathodes and it could just sound like mush or push the output stage too much. I could do some fancy equation to work this out or I could just spend 5minutes trying it .

Just worked out how the boost circuit on the JCM1 works, however I don't think its an option for the JMP1. I find it very odd how the JMP1 sounds pretty similar to the original JMP1 amps when its a single ended amp. Although you can still hear single ended character in the sound, its much better than a lot of the Single Ended Plexis that I've heard.

[billyoung]

gone

[statorvane]

Mr. Young, are you a bot? I have a hard time following your logic, thread relevance.

[xtian]

Mr. Young, are you a bot? I have a hard time following your logic, thread relevance.

Turing Test, please.