Paging RG Keen/UL700 pcb/Question about your BTDR2 Board and Footswitch

[Dennis Perusse]

Hello all,

Not 100% sure where this would fit in terms of forum on this site. It could fit on the Vox/Highwatt forum, the pcb forum and this one the Technical Discussion. I purchased RG Keen's UL700 pcb pre-covid whilst working on doing a vox ac10 using the 1960's circuit and using a contempo chassis. I finished that project now I plan on building a UL710T instead of the UL715 amp which I was originally going to do. For those who may not know the UL series of amp were a Hybrid amp which had a Solid State Pre-amp but a Tube Power amp. This series of amp had a few configurations up to 120 watts. R.G. has done a fantastic job of recreating the pre-amp portion of this amp and also supplied a booklet on it. He also has a way to replace the original reverb with a Belton BTDR2 unit and he also had a pcb which I purchased as well.

Now, the pre-amp board I have populated according to his Bill of Materials albeit a 12 volt .75 watt incandescent bulb which I just found today and have purchased from McMaster Carr. My questions, alas doesn't quite fit here, is in his BTDR2 pcb board and of the footswitch. I hope that RG replies to this as I have sent him a private message but where I am unsure if I sent it correctly but if anyone who has built one of these amps have done so I'd love to hear from others as well on their solutions. Here are my questions that either he or someone who have build this amp before might be able to answer...

1. The Foot switch:

The foot switch on the page that has the BTDR2 schematic shows a resistor and a transistor in the circuit in the Distortion switch. Their is also a three LED's present. My questions on this are what LED's are okay to use in that circuit, plus what is the resistor value as well as what Transistor is supposed to be in there that looks like it does the switching in the distortion section?

2. The BTDR2 board:

I purchased the BTDR2 board from you when I purchased the UL700 pcb from you. Do you have a schematic on what parts populate that board as you sent me the bare board. Also on your mod diagram which allows the reverb unit to be used on both channels what is the value for R1000, C1000, plus what is r1001 in terms of chip? Is it a TL072 at all?

For everyone who is reading this is very vague and probably no one will understand what I am asking about. Those who have built this amp with his pcb will know exactly what I am asking about. I apologize for how I am wording these things as it will make no sense to anyone reading this. Once RG's site on the Thomas Organ repair parts went away locating him proved very difficult for me. I am hoping that he responds to this in this manner. Thank you all of putting up with my question on here and I thank you all for reading this. I also want to say for those who have build this style of amp I appreciate any help in any capacity on answering these questions as well.

Dennis

[R.G.]

I'm on the road, away from my machine with the design stuff for those. I should be back in quarters in a couple of days. I could opine something now - like the LEDs being customer choice - but it's been long enough for those that I don't still have the details in my head.

Edit: I should note that the Thomas Vox repair site vanished when GoDaddy changed their house website builder so that the site no longer worked and has to be rebuilt from scratch.

[Dennis Perusse]

Hello,

Thank you to RG for answering my post so quickly, and for any and all who had read this post. Very much appreciated. If you can I put my email in the Private message so let me know if it did work and got to you. I have a few other questions that I would like to ask that doesn't pertain to this project but would love your input on as I had purchased two of those ul700 pcb's and the last one I have an idea about using but in a different way. I may end up buying another one of those reverb pcb's from you but depending on the idea that might change.


In short much appreciated to everyone.

[R.G.]

OK, back off the road.

I don't see your email in my archives, so could you send an update with all your questions to date so I don't miss anything. I've located the design documents folders on my backup server.

Now, the pre-amp board I have populated according to his Bill of Materials albeit a 12 volt .75 watt incandescent bulb which I just found today and have purchased from McMaster Carr. My questions, alas doesn't quite fit here, is in his BTDR2 pcb board and of the footswitch. I hope that RG replies to this as I have sent him a private message but where I am unsure if I sent it correctly but if anyone who has built one of these amps have done so I'd love to hear from others as well on their solutions. Here are my questions that either he or someone who have build this amp before might be able to answer...

I'd love to hear from others that may have built this board, too. I like mine, but I'm well aware that ears are different.

1. The Foot switch:
The foot switch on the page that has the BTDR2 schematic shows a resistor and a transistor in the circuit in the Distortion switch. Their is also a three LED's present. My questions on this are what LED's are okay to use in that circuit, plus what is the resistor value as well as what Transistor is supposed to be in there that looks like it does the switching in the distortion section?

The extra resistor and transistor is there only to manipulate the LED showing the distortion is on. The distortion switching in the original circuit is funny, needing pad 15 to be pulled up by a resistor to +14V -and- pad 16 pulled to ground by another contact at the same time. It confused me when I first went through it, as witness the corrected distortion footswitch wiring fixup.

The added resistor and transistor parallel onto pad 15, which is pulled up when the distortion footswitch enables the distortion. The added transistor turns on and pulls the distortion indicator LED on. 10K ought to be fine for the resistor. It's what I used. Too small a resistor there might make the distortion switch on incompletely. 10K seems to work.
A 2N3906 or 2N5088 should work fine for the transistor, but since the base resistor needs to be relatively high value, the 5088 might be a better choice.

The indicator LEDs are as I thought - use whatever you like. They're driven from 25V, which is wasteful of the voltage, but gives you enough headroom to use any LED you like. The 2.2K resistors are a starting point. If you use super-mega-hyper efficiency LEDs, you may have to raise those resistors up to 4.7K to 10K to keep from putting colored spots on the ceiling.

2. The BTDR2 board:
I purchased the BTDR2 board from you when I purchased the UL700 pcb from you. Do you have a schematic on what parts populate that board as you sent me the bare board. Also on your mod diagram which allows the reverb unit to be used on both channels what is the value for R1000, C1000, plus what is r1001 in terms of chip? Is it a TL072 at all?

I'm having a bit more trouble finding the archived stuff on the BTDR board. Some of the stuff I remember from looking at the schematic. Use 10K for both R1000 and R1001. C1000 is there to block DC in a more general application. In this use of the board, where you leave TR14, R17. and C44 off the main PCB, and change C43 to 220nF, C1000 can be short circuited. C43 at 220nF and R1000 at 10K gives a low frequency rolloff of 72Hz. You might even want to reduce C43 to 100nF or 47nF if the reverb sounds "muddy". Reducing the value of C43 (or C43 + C1000, effectively in series) will cut some of the lows from the reverb and cut some "mud" out of the sound.
R1001 is actually the feed back resistor for the IC, not the chip.
The TL07x series works fine there, but I have to find the layout to see if I used a TL071 or TL072. Stay tuned to this channel.

[R.G.]

I think part of my issue is that I need to know exactly which board you have. Can you email or post a picture of the component side of your BTDR board. I think I know which one, but need to be sure before I run you down alternate garden paths.

[Dennis Perusse]

R.G.


Sure, I can send it but do you want it to your email or on this forum, or both?

[R.G.]

Email I think. Avoids cluttering up the forum, and if someone is interested, they'll ask.

[Dennis Perusse]

R.G.

My email address is in the private message that I sent. 1. Did you receive it? If not then I'll post it here for you just spaced so the bots don't send stuff to it.