Are these preamp voltages OK?

[titser_marco]

Hello! I'm still in the process of tinkering with my D-style build and I figured I'd take a voltage reading in the preamp as that seems to be the source of my complaint about my build, ie general harshness in the OD:

V1 B+ before 100k plate load:322
V1 plate voltage (after 100k plate load): 205vDC

V2 B+ before 100k plate load:325
V2 plate voltage (after 100k plate load): 206vDC

Are these OK?

Also I'm using some freshly reconed CTS speakers from a Twin. I thought I'd put that in just for information's sake in case the harshness is because of the speakers.

[Stevem]

The voltage on the pramp tubes in and of itself is not sourse of harshness unless you run the tube(s) way up near there rated max voltage, and at that point they sound brittle even in clean mode.
This does not hold true for PI sections where many are fed much higher voltages.

I find that if you have anywhere from 180 to 220 volts on the plate of a ax7 that I am good with the needed voltage gain and tone in both clean and clipped modes.

Some tubes themselves go into a brittle sounding mode when pushed into clipping, this takes place a lot in Russain made preamp tubes and even more so in there 5881/ 6L6gb type tubes, in the NOS 6V6 world GE brand tubes are the brittle king unfortunatly.

A new reconded speaker can be soft in the lower mids and the bottom end until the Surround and Spider breakes in, and this can make for harsh highs .

If you go to the uncle Spot .com site you can see the recommend 60 hz AC voltage to apply to a new speaker to break it in.

On a speaker with treated Surround I run them for 20 hours, on a non treaded Surround I go for 25 hours .

You may first want to try your amp thru a friends speaker(s) and then note any improvemnt .

Your issue may or may not be helped by adding a 250 to 500 PF cap across a plate of the last gain stage , but make sure it will handle your power supply voltage at that node!

Your issue may also lie in the output stage and you might want to read up on
Paul Rubys on line posting about crossover distortion and how to kill it.

[martin manning]

Marco do you have the FET booster circuit in your amp? If not put a 150k 2W resistor going to ground on the last power supply node to simulate it's current draw. That will lower the preamp voltages a bit. The consensus here is that V1's plates should be at 180-190V.

[titser_marco]

Marco do you have the FET booster circuit in your amp? If not put a 150k 2W resistor going to ground on the last power supply node to simulate it's current draw. That will lower the preamp voltages a bit. The consensus here is that V1's plates should be at 180-190V.

No FET circuit. Re: lowering voltages, would it be better to just change the value of the dropping resistor for V1's plate supply, or change the load resistors instead?

[martin manning]

Keep the plate loads the same. I would add the 150k to ground.

[titser_marco]

So I tried a 120k just now as I didn't have any 150k in stock. The effect on the harshness was minimal, but I could definitely hear the overtones more. If this were any louder (11pm in Manila now, living in a condo building) I could imagine calling up feedback at any pitch in this set up. I just might try and lower the V1 and V2 voltages to about 180-185.

In any case, I'm starting to think whether I am just expecting the wrong things from this amp. I can't take the harshness it has when Presence is maxed but I like what the amp sounds like with that control dialed down. Is it unrealistic to expect themsame smooth D-style overdrive when the Presence control is maxed?

[JazzGuitarGimp]

It could be an ultrasonic oscillation is springing up when you turn the presence up. Do you have an oscilloscope you can yse to look for oscillation? If not, you might want to try repositioning or shielding the negative feedback wire run.

[Structo]

The 150K resistor should lower your preamp voltages slightly.

Your voltages aren't too bad but you generally want around 200vdc on V1.

Best place to probe is on the eyelet on the board that goes to pin 1 or pin 6.

I don't understand why you want to run the presence so high?

Do you have a bright cap on the Master volume? On the input volume?
Mine is generally set to 1:00-2:00.

My amp has high plate resistors. (220k and 150K)

I have around 199vdc on V1 and around 205vdc on V2 plates.

The PI has around 285vdc and 295vdc. (can be higher)

Different tubes will draw different amounts of current so the voltage can vary a bit.

You can change the dropping string resistor values on the power supply to
move the voltage up or down to fine tune it.

[tubeswell]

Depends what kind of amp you're aiming for. 3 hundy is good for a cleanish jazzy sounding preamp or a bass preamp. Even a Fender Pro 5E5 schematic shows 300 HT for the pre-amp

[titser_marco]

It could be an ultrasonic oscillation is springing up when you turn the presence up. Do you have an oscilloscope you can yse to look for oscillation? If not, you might want to try repositioning or shielding the negative feedback wire run.

I don't have a dcope so that's a tricky part. I will try your suggestion to shield the NFB wire.

[titser_marco]

The 150K resistor should lower your preamp voltages slightly.

Your voltages aren't too bad but you generally want around 200vdc on V1.

Best place to probe is on the eyelet on the board that goes to pin 1 or pin 6.

I don't understand why you want to run the presence so high?

Do you have a bright cap on the Master volume? On the input volume?
Mine is generally set to 1:00-2:00.

I have a bright cap at the input volume, but I rarely turn it on. Maybe you're right about the Presence being set too high. I guess it's just me thinking that all positions of this amp's controls should be giving a usable tone.

Would you guys know of a clip where a D-style amp's Presence was maxed? I'd like to be able to compare, as a part of me thinks that I may be overthinking it. What do your builds sound like with the Presence control maxed?

[Stevem]

The presence/ feed back circuit in any amp cuts the gain ( kills off some feel, volume and dynamic range) but it also extends the frequency range on both ends of the spectrum, and doing so above 5.5 kHz can make for harsh clipping tones when played loud until the feedback drops off.

It also makes the clipping and harsh tones come into play sooner due to the point in the amps output where the feedback drops off the cliff and the amps distortion kicks right in!

You might first want to try hooking the feedback lead to lower impeadance tap on the OT for less feedback voltage, or install a resistor in line to cut the voltage by half and see what that does for you.

[martin manning]

This thread really belongs in the D forum.

The Presence control is targeting high frequency feedback, so lowering the presence is one way to reduce high frequency content. The output section is basically a clean power amp, and I would recommend keeping it as Dumble intended. If you want to reduce highs some, one way is to play with the bright caps, but they are only effective at reduced volume settings. Increasing the value of the snubber caps on the OD is another way, and that will be effective at any volume. Lastly, many people have had success adding a treble bleed network at the output of the OD section. You can find info on that here using the search function.

[titser_marco]

This thread really belongs in the D forum.

The Presence control is targeting high frequency feedback, so lowering the presence is one way to reduce high frequency content. The output section is basically a clean power amp, and I would recommend keeping it as Dumble intended. If you want to reduce highs some, one way is to play with the bright caps, but they are only effective at reduced volume settings. Increasing the value of the snubber caps on the OD is another way, and that will be effective at any volume. Lastly, many people have had success adding a treble bleed network at the output of the OD section. You can find info on that here using the search function.

Happy to have the mods move it to the D Forum, or wherever it is more appropriate.

In any case, I tried putting in a treble bleed cap (.001uF) in the OD section and it seemed to work, but I played around with some other values and it seems that 500pF is still too much, while 250 isn't. I'll probably get myself a 330-390pF cap and try it there.

Long and short of it is that I'm able to max out the Presence and still have a usable tone that isn't too bright, but more than enough to cut through a really thick mix. For the most part though, the usable range is between 1-3 o clock, and below that there is very little change that can be heard. Note though that used a 5k pot there as I had no 2k-2.2k stock at the moment. Parts can be very tricky to get here in the Philippines, much more so get the exact value that you need. In any case, I just might settle for a 2K pot.

I also moved the NFB wire from the 16ohm tap (what was I thinking?) and put it on the 4 ohm tap. Additional improvement gained from that as well.

As regards the snubbers, I don't think I'll try putting them because right now I already have 500pF snubbers on both OD gain stages from plate to cathode. I'm actually thinking of taking them out (or reducing them, say to 25pF just like the 183 schem) after I put in the treble bleed and see how that sounds.

I'll see if I can record clips and send over to you guys. Thanks for all the help and for being patient!

[martin manning]

Put a 3k3 across your 5k pot and you will have 2k. What you describe is the pical presence control behavior.