EF86 preamp in deluxe Reverb

[Meddon]

Hi

I have a AB763 Deluxe Reverb kit where I built the EF86 preamp from a Dr. Z Route 66 in the Normal channel. Here is the layout I used: Dr. Z Route 66 Layout | Flickr - Photo Sharing!

The problem with this build is that the EF86 preamp is a lot lower in volume than the Vibrato channel, and with a Tele it won't break up even with Vol and both tone controls at 10. The Vibrato channel breaks up nicely with the volume turned up.

Should I reduce the Normal channels 220k mixing resistor before the PI, or is there some other tricks to get more juice from this channel?

Thanks!

[jrfrond]

Please post the preamp schematic. There's more than enough gain available in an EF86 to not have volume discrepancies.

[Meddon]

Here's a link to the schematic: http://www.flickr.com/photos/4910811...7895/lightbox/

[tubeswell]

Hi Meddon

The R66 schematic you posted doesn't tell us how/whereabouts you spliced the pre-amps together. Got any more info?

[Meddon]

Hi Tubeswell!

I soldered a wire from the point in the R66 schematic where the wiper on the volume pot and one leg og the .0068 cap come together. This wire goes to the point in the AB763 layout http://www.ampwares.com/schematics/d...verb_ab763.pdf marked X, just before one of the two 220k mixing resistors.
Should I have included the .047 cap that comes after the preamp on the R66 schematic?

[tubeswell]

Hi Tubeswell!

I soldered a wire from the point in the R66 schematic where the wiper on the volume pot and one leg og the .0068 cap come together. This wire goes to the point in the AB763 layout http://www.ampwares.com/schematics/d...verb_ab763.pdf marked X, just before one of the two 220k mixing resistors.
Should I have included the .047 cap that comes after the preamp on the R66 schematic?

The 22nF coupling cap blocks DC from the EF86 plate. The 47nF before the LTP blocks DC from the tail resistor of the LTP from getting onto the tone stack. Therefore I wonder whether not having the 47nF could be affecting the impedance bridging. Why don't you put it in and see?

[Meddon]

I think adding the .047 cap may have helped a bit, but the EF86 channel is still cleaner and less loud than the vibrato channel. I measured some voltages, and they seem very low to me, but I have never used a EF86 before:
pin 1: 76 volt
pin 8 & 3: 2.17 volt
pin 6: 65 volt

Is it an idea to try and raise the voltages to pin 1 and pin 8/3 by lowering the resistor values (1M and 220k) at the voltage divider?

[tubeswell]

I think adding the .047 cap may have helped a bit, but the EF86 channel is still cleaner and less loud than the vibrato channel. I measured some voltages, and they seem very low to me, but I have never used a EF86 before:
pin 1: 76 volt
pin 8 & 3: 2.17 volt
pin 6: 65 volt

Is it an idea to try and raise the voltages to pin 1 and pin 8/3 by lowering the resistor values (1M and 220k) at the voltage divider?

Check that you have an actual 220k going to Pin 6 (plate) and an actual 1M going to Pin 1 (screen). Check also that you have 0.1uF for the screen bypass cap.

Pin 8/3 are the cathode and the suppressor grid. Those voltages should be low.

And what is your HT voltage at that Pre-amp node on the power supply? (It should be around about 300-350 in order to get the Route 66 voltages)

[Meddon]

I checked the resistors and they're ok. The 0.1 bypass is also ok.
But there is only 250 volt at the preamp node.

[tubeswell]

there is only 250 volt at the preamp node.

Reduce that pre-amp supply resistor and see how it goes - try putting in one half the size to start with.

Even so the stage still should have heaps of gain. I am wondering about the coupling cap after the plate - have you measured it for DC leakage?

[imaradiostar]

An EF86 straight into the phase inverter should be REALLY loud- I have a similar amp and it sounds great with lots of gain. Try lifting the 220k coming from the trem side and see what happens. The Rt66 tone stack relies on the really high input impedance of the phase inverter. A 12ax7 can drive the mix resistors much harder. If anything you want to increase the values of the mix resistors so that the EF86 channel has a fighting chance of being able to drive a higher impedance.

By the same token- why not mix the EF86 channel in with a 470k resistor after the 3.3M reverb blend resistor?

In my experience the rt66 pre is at its best in a super simple amp with no other channels or verb or anything, like the other Z amps in that series.

jamie

[Meddon]

That's very interesting!
I assumed that lowering the value of the mix resistors would give yield more gain into the PI, but it's never that easy, is it?
I'll do some surgery on the amp this weekend and see how it responds to higher input impedance of the PI.

Once the amp is working properly, I had originally planned to run the EF86 channel through the reverb and vibrato circuit by mixing the channels before the effects, but maybe that isn't such a great idea. The reverb I can live without, but tremolo on both channels would be nice. I guess I could also convert the amp to bias tremolo, which I personally think sounds nicer than bug trem.

[imaradiostar]

I agree on the bias vary trem- always seems to sound better to me. Have you seen the schematic for the Dr. Z Z-28 that's floating around out there on the web? It has slightly different tone values than the Rt 66 which might work better with the 6v6's. Also the phase inverter is different, perhaps to complement the distortion tone.

My main amp is a Rt 66 style with 7591 output tubes. I added my own single tube verb design. It sounds pretty good but I had trouble shaking a strange oscillation so I never use it. I would tend to favor a single tube verb (like the one on that's popular on the EL34world site) with that preamp rather than the fender design. It seems to work better with High-Z preamps.

I'm curious to see where you land!

jamie

[Ben N]

Don't EF86s have higher Z-out than typical 12a_7 preamps? Maybe a buffer stage, such as a mosfet follower, would take care of your problem.

[wsaraceni]

im thinking of going with a similar style amp myself. i was wondering how yours finally worked out. I was thinking of going z28 preamp into a trainwreck express/marshall poweramp with a 300-0-300 transformer. would definitely like to hear opinions on this though.

[clyde1]

Never had much luck with EF86's in combos because of microphonics and vibrations. I hope yours turns out better.

[isaac]

ef86's are temperamental, could be a bad tube. Also, I noted your your voltages are about correct for a generic ef86 application. I'm not sure about the RT66 in particular but in general they look good.
Also, check your connections and component values throughout the tonestack, those could be bad, or just bad solder joints, you never know.

[isaac]

You may also have the pins 1 & 6 switched, or at least their resistor feeds. I see you have the higher voltage on pin 1 when it should really be at pin 6. Pentode tube voltages can go somewhat high, but the screen always has to have a lower voltage than the plate, at least a little bit. So, my bet is that you swapped the B+ feeds for pins 1 and 6. Can anyone +1 this?

[imaradiostar]

Just to throw a wrench in the works- you may want to try a 6au6. They seem to work well with lower plate resistors, say 82k to 100k. You should end up with a screen dropping resistor about 2.5 times the value of the plate resistor for a 6au6 based on the current ratios shown on the datasheet. I'd probably start with 100k plate and 270k screen or 82k plate and 220k screen and see how it sounds.

Jamie