JTM50 Black Flag PT dual voltage question

[skidbrake]

Greetings,

I'm building a 50w black flag spec with a tube rectifier. For the PT I'm trying a ClassicTone 1202-118 replica (along with their version of the 784-139 OT) and see that this PT comes with dual secondary voltages: 690vdc @150mA and 420vdc @150mA.

I'm assuming that by wiring it for 420vdc it will make the amp warmer, with less headroom, which is what I want. But I'm concerned that 420v may not give the pre tubes what they need to run efficiently.

Here is my as-built and trannys:

Schematic: http://www.7b5.me/amps/BF50.pdf
PT: http://www.7b5.me/amps/PT.pdf
OT: http://www.7b5.me/amps/OT.pdf

Am I OK wiring it this way, or should I stick with 690vdc as the schematic indicates?

Thanks for your advice, much appreciated.

[Roe]

420v is ok with a 8.2k droper

[skidbrake]

Thanks Roe. Am I correct that using the lower B+ will give the amp breakup at lower volume?

[Roe]

yes, more compression and earlier breakup. and more mids

[skidbrake]

I'd like to wire the amp for both high and low voltage on the tube rectifier, replacing the standby switch with a 3PDT for dual voltage, using a layout I found on this forum.

http://www.7b5.me/amps/dual.jpg

... and using this switch, which used to be available from Metro (hopefully I can find it from Newark or Mouser):

http://www.7b5.me/amps/switch.jpg

The only thing I wonder about is the value of the dropping resistor. In the circuit I'm building (see link above) the drop resistor is 220K. Roe mentioned using an 8.2K drop when running low voltage, which is a long way from 220K.

What if I were to wire the board for low voltage, using an 8.2K drop resistor on the diode (instead of the specified 220K) and use a separate 250KL pot as the high voltage bias pot, wired across the switch (pins 3 and 6, according to the diagram) to dial in the high voltage bias? Would that work?

Thanks for your help.

[skidbrake]

I emailed George to see if he still sells dual voltage switches. If not, I found a 3PDT [on-none-on] Carling switch on Newark that should work:

http://www.newark.com/carling-technologies/hl251-73/switch-toggle-3pdt-15a-250v/dp/73K0988?Ntt=carling+hl251-73

[Roe]

are you talking of the 220k bias dropper? I'm talking of the 8.2k preamp dropper!

[skidbrake]

Oh ... my misunderstanding. NOW this all makes more sense. I am learning. Thanks for your advice.

[arledgsc]

Schematic: http://www.7b5.me/amps/BF50.pdf

Hey that is my schematic That design took me better part of a year to tweak up but quite pleased with the current results. I added more B+ filtering consistent with other BF designs and many, many other changes. The biggest problem was hum from the PT coupled to the OT. I had to rip up the board and physically move the OT away form the PT. It is a Ceriatone chassis which places the OT very close to the PT versus other chassis. The amp is currently my #1 player.
Below is the latest schematic with all the changes. YMMV

WARNING: The OT primary wires are reversed in the orginal schematic above. Corrected below. (Yes, I wired it wrong initially )

http://www.keepandshare.com/doc/1782400/jtm50-schematic-custom-spec-a-14-68k?da=y

Anyway, how is the 690V B+ working out for you? Seems like a lot for a 50 watter considering modern tubes. Thanks, Scott

[skidbrake]

Hi Scott,

Thanks for generously sharing your schematics and ideas. I was inspired to build this amp because of quality of your work.

I built the amp using premium components and it is, without a doubt, the best sounding amp I've ever played. The cleans on it are buttery and bluesy, and the bass freqs are tight. On the low voltage (420v) setting, it starts to break up at Vol 4, and if I jumper the channels it becomes a rock monster.

I prefer the 420v setting to the 690v, as I don't need the additional headroom for my usual style of playing. When I want to play jazz with an arch top, I'll switch to 690v which seems to add a bit more space to the tone. Tubes are Winged =C= EL34s with JJ 12AX7s. I tested Mullards, Groove Tubes and Tung-Sols on the pres, and found the JJs sound the best. I mainly play Strats with vintage-spec pickups, with mostly clean tones at the edge of break up. This amp seems custom-made for this style.

In my earlier posts I was trying to sort out how to tune the voltages, and ended up using a 22K drop resistor in place of the 8.2K. That brought my plate voltages close to spec on both settings. I also used separate bias pots, so I was able to dial in for both voltage settings.

I used a steel Ceriatone chassis and had no noise issues with the tranny locations. Like you, I'm a believer in the Larry Ground technique, and find it works so well I no longer shield the input leads.

For transformers, I went with ClassicTone's 50W Drake replica offering. I haven't used their iron before, and the low price worried me a little, but they really deliver. http://www.classictone.net/Marshall-Style-Transformers.html#anchor_1199 Their PT is wired for high/low voltage, which is why I decided to figure out how to wire both into the circuit through a switch.

Peter Mather in Nashville http://www.mathercab.com/price_list_marshall.htm built me a great BB 2x12 cab (his cabinet work is the best IMO). I spoke to Jim Seavall at Scumback about the amp and he recommended 65-watt M75 and H55 speakers, which sound great in this application. http://www.scumbackspeakers.com/m_series.html By going with the 65-watters I can also plug a 100-watt head into the cab later if I want to. (I'm now building a JTM45/100 head)

I'll post some pics soon. Again, Scott, I want to express my gratitude to you for sharing your research and best practices with hobby builders like me. It's great to be able to build something this good, and that would not be possible without folks like you.

-Mike

PS - I caught the OT wiring mistake you mentioned ... the old fashioned way. It was midnight, of course, and the foghorn blast awakened my entire household.

I see you added an FX loop to your schematic. How is that working? Any noise issues with it?

[skidbrake]

Scott, here is the final voltage chart, FWIW:

http://www.7b5.me/amps/JTM50BF_Volts_Final.pdf

[Roe]

preamp voltages are low, too low IMHO. are you using a double droppers? a single 8k2 is sufficient

[skidbrake]

I'm using a single 22K 2-watt decoupling resistor (power dropper at the NFB circuit).
At the preamp filter, there is a 8.2K under the dual cap, and a 10K across the cap (+) leads.

Roe, are you saying I should remove the 10K and just use the 8.2K?

[arledgsc]

Thanks for generously sharing your schematics and ideas. I was inspired to build this amp because of quality of your work.

Thanks but all credit goes to the original designers, Larry, Lar/Mar, MetroAmp, Marstran, etc. I just documented my build as best I could with what I had. Comes in handy when troubleshooting and for mods. If it helped you out then I am pleased!

The preamps voltages do look low. What values are you using for B+ dropping resistors?

The FX Loop Return could use a little extra gain (maybe other 10dB) to push power amp to full screaming output plus some headroom. In the home studio it is fine. I'm using a TC Electronics G Force and with its noise gate so there is little to no noise or hum with it in the FX path. Reverb tails off into silence but not until after moving the OT away from the PT. That coupled hum problem drove me crazy for 6 months. Sounds super now and quiet.

Glad the Scumbacks are working out for you. I have a pair of M75 and M55 65W on order and hopefully will ship soon.

Thanks again and glad you are happy with your amp. Sorry about the reverse OT primary but I too had the foghorn from hell initally. Cheers, Scott

[Roe]

I'm using a single 22K 2-watt decoupling resistor (power dropper at the NFB circuit).
At the preamp filter, there is a 8.2K under the dual cap, and a 10K across the cap (+) leads.

Roe, are you saying I should remove the 10K and just use the 8.2K?

I'm referring to the power resistor to the left of the nfb resistor