Metropoulos Forum

I'm using a single 22K 2-watt decoupling resistor (power dropper at the NFB circuit.

Skid, if you like the sound of your amp then leave it alone. Low B+ though may cause clipping at high sound levels and produce undesirable distortion if the signal clips. Modern 12AX7s can't handle more than 300V plate voltage tops so I shoot for 300Vdc at V2 pin 6 since it is connected directly to B+ supply. Most design aspects are a compromise so a few volts over or under is OK.

With 22K PI, 8.2K V3-V2, and 10K V2-V1 B+ dropping resistors I would be concerned that the V1 plate voltage may droop from its already low value and take a while to recover since electrons are pushing through 40.2K ohms to charge the relatively large 33uF cap at V1. Plus the JTM50s are heavily filtered so you have other caps to charge as well down the B+ chain before V1. If you feel a need to experiment try 10K PI, 10K or 8.2K V3 to V2, and 2K V2-V1 dropping resistor values for the high B+ setting. You have a desirable B+ voltage of around 450V on the EL34 plates so it is matter of selecting dropping resistors for the earlier stages. If everything is linear on the low setting of 400V for EL34 plate voltage the preamp tube voltages should drop about 12% from the high setting.

Anyway, its all rock and roll and early breakup, grit may be very desirable. If you are happy do nothing. All the best, Scott

You raise a very interesting question: I don't know if I have a clipping issue, as I don't crank my amp. I'll test some of the different resistor values you suggested and see if I can raise V1 voltage. I'll report back soon. Thanks for the guidance.

I was thinking the whole preamp and pi, not just v1.

On the high voltage setting, I found that a 15K PI dropper gives 292v to V2 pin 6.
I started with an 8.2K (330v) and a 10K (323v) and kept raising the R value until it fell below 300v.

I changed out the V2-V1 dropper to 5.6K to bump the voltage on V1 pin 1 to 195vdc. Maybe that's still too low?

The tone sounds much the same; perhaps a little bit more glassy chime, which is nice. The biggest difference is that the volume is more linear. Before the amp would get loud above Vol 3; now it goes up gradually. I turned it up to 10 and it blew the H.T. fuse. Not sure if that was random or not.

Here is how the voltages look now, on both high and low settings:
http://www.7b5.me/amps/JTM50_volts.pdf

I wouldn't be afraid of going a little above 300v really as long as you're not using the new tung sol tube or some of the sovteks

The rectifier tube is a 5AR4/GZ34 Sovtek. What do you think is the best brand of rectifier tube for a 50W Marshall? Mullard? I was thinking of trying some different ones to see how it effects pick attack and headroom.

I was referring to preamp tubes. the mullard is best since it lasts for a long time and has little sag. personally, I'm using a ss plug in rectifier in my jtm50

I tried a Weber copper cap in the rectifier position. It makes the amp a little louder and brighter.

Skidbrake could you possibly send me the way you hooked up your dual voltage switch? I’m trying to build a jtm50 as well and would like to have the high and low voltage options on the standby switch. I have a 3pdt switch and a correct PT for this but I’m having trouble wiring it up as I still have a lot to learn! Any help would be greatly appreciated!

Hi John,

It has been a while since I worked with this amp, so I looked in my As Built archive. Here is how I wired the dual HV/LV switching. The image uploads on this forum are not working correctly, so I uploaded to my server ... hit this URL to download it:

https://michaelcoffin.com/50w_dual_volt.jpg

Hope this helps. Good luck!

Mike

@ John_tucker66...did the link to the layout I posted for you in the "transformer and choke" subforum help any? I thought that the low/hi voltage switching was covered in those 1959 layouts on SDM's website. Make sure that you fully understand the wiring of the switch and be sure to use a light bulb limiter when you do your first power up. A mistake in this part of the circuit can easily damage an expensive power transformer unless the LBL is used. It may be best to build the amp stock with the plan of adding the switching mod after the amp has been powered up successfully for the first time and you're sure that it's working properly. Trying to troubleshoot a new build with extra modifications can really complicate the situation.

Skidbrake! Thank you so much that was very helpful! I really appreciate it.

danman, I definitely saw the link you sent and that was very helpful as well! I was unsure at first, being a novice, because the 100 watt circuit already has the secondaries right up to the standby switch, jtm50 has the secondaries straight to the rectifier like a jtm45 and the standby lifts the DC voltage between pin 8 on the rectifier before the mains filter cap, so basically I really needed to understand that where the switch is placed in the circuit is a different way to lift the voltage and create the standby function. This really forced me to understand the flow of everything in this part of the circuit and I learned so much! I couldn’t have done it without your guys help and also immersing myself into understanding exactly what everything is doing, wiring wise, because I would not even think about turning the amp on without being absolutely sure everything is correct. I definitely did not want to ruin a brand new PT! Anyways, I got the amp fired up yesterday and it is glorious!! I’m so happy that I have the dual voltage switch in place too because I love both high and low voltage settings for different reasons, and it will be very useful for recording as well. Thank you guys so much!

Happy to hear that you got it finished up and working properly. Definitely a good thing taking your time and understanding how the circuit works. Congratulations on a successful build!

Well done!

Hey Skid. Nice to read your thread.

Do you have any layouts/schematics for the JTM 50 "Black Flag"? I'm interested in building one myself.

Thanks, Don