FET input question & more

[bluemonk]

Hello. I almost finished my standalone D-preamp and I'm testing it connected to the power amp of my DIY Hiwatt. I've a pair of question and hope in your help.
- FET input seems to be clipped by a Strat also at low trimmer setup. I understand it's intended for low-signal piezo but I'd like to get more control. I followed #124 schematic and used BF245A. Any advice?
- Rock-jazz switch IMVHO cuts excessive bass and overall volume. How to make it smoother?
- YT videos shows real-D and clones to be able to infinite sustain. Not mine. Some suggestions to tune it up?
Many thanks for your attention.

[bluemonk]

Update:
solved the FET issues thanks to the 2-batteries-trick found in an old thread.
BF245A asked for only 240R source resistor to get 9V at drain and 17,5V supply. Perhaps it's a weird value but seems to work.
Still puzzling with the other questions and hope in your advice.
Thanks in advance.

[dcribbs1412]

Hi bluemonk
glad you have the FET working,
if possible post some pics and schematic you used,
which may help others suggest options in the tuning process.
Lots of info here, the search function works well.

D

[ayan]

Hi,

- Rock-jazz switch IMVHO cuts excessive bass and overall volume. How to make it smoother?
- YT videos shows real-D and clones to be able to infinite sustain. Not mine. Some suggestions to tune it up?
Many thanks for your attention.

As far as your first question goes, yes, the R/J switch will lop off the bass and result in a pretty noticeable drop in volume. I believe that is the reason hardly anyone uses that setting at all and many leave the feature out all together when they build amps. I'm afraid it's one of those "it is what it is" things. As you probably have noticed, however, the R/J switch is involved in the wiring of the preamp boost function, so perhaps you may just want to leave it alone and not use it.

Regarding the infinite sustain, I am not sure whether you mean that your amp doesn't have "gain" or that it doesn't sound "smooth with 'cleanish' sustain." If the former, a couple of things for you to check out:

1. If you set the preamp volume all the way up and turn the tone knobs all the way up, you should get a little bit of "grind" on the clean channel when using humbuckers, for example. Is that the case for you? If not, then check the wiring for the clean channel, to include the tone stack, the R/J switch, the preamp boost, etc.

2. Assuming the clean channel checks out OK, considering you used the schematic for 124, you have a trimmer inside the amp that should measure about 350K between the 2nd clean and 1st overdrive stages. Personally, that value is high for my taste and will result in a dark/duller sound; I favor the more typical 100K trimmer value. Regardless of that, how high do you have that trimmer turned up? One way of measuring it would measuring the resistance between the wiper connection and ground. I would turn it up about 30-40% and see what that does for you. And again, I would encourage you to experiment with the 100K value.

3. If #1 and #2 check, how do you set your overdrive "gain" control? If you use and audio pot there, you will want for that to be up around 2 o'clock or so. If you use a linear control, about 11 o'clock would be good. With these settings, and the preamp volume also at about 2 o' clock how much overdrive do you get? There should be plenty, and there should be quite a bit more if you engage the preamp boost, assuming you set your tone controls midway up or so.

4. If you don't get enough gain from the overdrive channel but the clean channel works fine, then you should focus on the trimmer at the entrance of the overdrive and the pot between the first and second overdrive stages. Make sure the values of all components (plate and cathode resistors, bypass and coupling caps, trimmer, pot) are OK and that the V2 tube (the one for the overdrive channel) is OK too. A quick thing you can do when checking things out is look at the plate and cathode DC voltages for all stages, plates should be around 200 VDC and cathodes 2VDC, + or - 20% or so -- this is a nominal check, if you for example should get for example a cathode voltage of .5 VDC, something would be off. You want to make sure you don't have bad connection or inadvertent ground somewhere, and the voltages will give you an idea.

Now, if your question is about getting sustain without the amp being excessively overdriven, then we're talking about component and tube choice, lead dress, orienting the coupling caps in the correct direction and setting the phase inverter trimmer properly. All of these topics have been discussed thoroughly in the forum and there is even a sticky for how to set the PI balance. This is the most elusive part of making an amp sound right, and I would think the majority of the posts on this forum have to do with this to some extent.

Hope this helps to get you started, hopefully you will report back and folks will be able to give you more specific ideas.

Gil

[bluemonk]

My preamp is still in progress but here are two pics. The schematic is #124. The layout has to be arranged (mirrored and rotated) to fit in the chassis I had at hand. I use salvaged parts as I can.
I have a couple of external PS for preamps, built into PC supply boxes. Filaments run on DC. Relays at 6V regulated by 7806.
But this preamp is well worth a dedicated toroidal PT. The tubes are 6n2p. I will try to swap to 12ax7 altough in other builds I was satisfied with them.
The layout is mostly from Ceriatone ods rev.2 because the white background helps, but values are from Ampgarage file. This mix of layouts drave me in a weird issue about the correct working of the Manual-Footswitch switch. Finally I realized that I had to lift the ground from OD switch and connect PAB switch to GND.
I used a BF245A in fet board because it was said to work correctly but in my exprerience it needs very different values with respect to the schematic. At B5+ of about 295V it needs 17,5k (not 8k2) after 150k to get 17,5V supply. And 240 source resistor to get 9V at drain. Hope this helps.

[ayan]

My preamp is still in progress but here are two pics. The schematic is #124. The layout has to be arranged (mirrored and rotated) to fit in the chassis I had at hand. I use salvaged parts as I can.
I have a couple of external PS for preamps, built into PC supply boxes. Filaments run on DC. Relays at 6V regulated by 7806.
But this preamp is well worth a dedicated toroidal PT. The tubes are 6n2p. I will try to swap to 12ax7 altough in other builds I was satisfied with them.
The layout is mostly from Ceriatone ods rev.2 because the white background helps, but values are from Ampgarage file. This mix of layouts drave me in a weird issue about the correct working of the Manual-Footswitch switch. Finally I realized that I had to lift the ground from OD switch and connect PAB switch to GND.

It kind of sounds like you fixed your problem. One unsolicited comment that hopefully will help you out. You're committed to the layout you have due to real estate considerations, but you don't necessarily have to use "significantly different parts" which may affect the sound: point in case, what appear to be metal oxide resistors on the plate loads and slope positions. A change that will set you back about $1 and may go a long ways. Your cathode resistors may be other than the preferred kind as well, I couldn't quite see them in your picture.

Gil

[bluemonk]

Hi Gil. First of all thanks for all the advices you gave me... once again (I'd never build anything without the "school" from you and other guys on Ampage and elsewhere many years ago
In fact I see that D-design is rich of tone nuances. I've to play and learn to use it. Voltages are correct, there's plenty of gain. I replaced the 250k OD trimmer with a 100k pot. That's ok.
Yes plate and cathode resistors are metal oxide. It is a temporary arrangement for testing and debug. As I said the work is in progress and I'm still waiting for parts I ordered (including two caps).
About resistors: are all plates and cathodes metal film R?
Or just in 1st stage?

[ayan]

In the great majority of all documented amps yes, plate and cathode resistors are metal film. Browse through some of the pictures and check it out. Slope and other resistors have also been metal film in some amps, carbon film in others. I use carbon films myself in those other tone positions.

Gil

Hi Gil. First of all thanks for all the advices you gave me... once again (I'd never build anything without the "school" from you and other guys on Ampage and elsewhere many years ago
In fact I see that D-design is rich of tone nuances. I've to play and learn to use it. Voltages are correct, there's plenty of gain. I replaced the 250k OD trimmer with a 100k pot. That's ok.
Yes plate and cathode resistors are metal oxide. It is a temporary arrangement for testing and debug. As I said the work is in progress and I'm still waiting for parts I ordered (including two caps).
About resistors: are all plates and cathodes metal film R?
Or just in 1st stage?

[Structo]

I have to admit that I have probably only plugged into my FET input a couple times.
But I wanted the full ODS experience, including Rock/ Jazz and the FET circuits.

This amp really made me embrace my humbucker guitars.