The Pentode (Preamp) Homage to HAD

[ElectronAvalanche]

Hi gang,

Based on the schematic provided by forum member Bombacaototal I set out to pay homage to the late Mr. Dumble:

The Pentode Amp (aka Silver Alligator) as a stand-alone preamp version. With Tremolo but sans Reverb (I have some ideas for this using a stand-alone studio grade Spring Reverb unit from Vermona.

The Tremolo part is based on a Vactrol design out of a DIY pedal kit from musikding.de (Das Tremolo) and inserts the LDR of the Vactrol right at the Master Volume. I only use the Oscillator part of the kit, which has the benefit of being able to set the shape of the oscillator to very smooth or choppy tremolo styles. The three pots on the back of the preamp control smoothness, shape and symmetry.

PT currently is a 290V / 40mA, 14V/2A, 6.3V/1,5A. Since the PT is slighlty to tall for a 1U enclosure, this needs to be replaced by a 270V/80mA, 14V 2A unit in the near future.

The EF86s heaters are powered by a standalone voltage regulator pcb with a 7805 (two 1N4007 in series with common pin of regulator give 5,8V under load). Which is within the specs for a EF86. Maybe I will switch to a 7806 to be closer to 6.3Vs. The heater of the 12DW7 is derived from yet another voltage regulator pcb with a 7812. This regulator also powers the tremolo oscillator.

Both regulators are fed from the 14V winding of the PTand have their own FWB recitifier. Both regulators are grounded via the 78s directly to chassis (acting as a heat sink as well).

The preamp sounds very nice as is, with nice chime and grit if driven hard. I will report back on voltages in a separate posting.

One problem I have is (probably related to the heater supplies):

I get a faint 100Hz hum (independent on preamp gain). In my previous HRM preamp I also had this due to too little filtering of the regulator (fixed by going up to 2200uF on the regulator input and 1000uF on the output side.

I wonder if somebody might have an opinion on the 100hz noise. Could it be that having two FWB from the same 14V widing is producing the ripple? I could of course take the unregulated rectified voltage of one of the pcbs to feed the second regulator pcb . The pcbs allow this via dedicated unregulated outputs. Or should I lift the 12V regulator from the chassis heat sink?

I am really an undergraduate when it comes to voltage regulator supplies. So any help is welcome. I will post a schematic of the heater supply.

Here are some pics of the preamp. The faceplate art will follow once the preamp is working 100% to my liking. i will also post some soundclips.

Best Regards,

Electron

[ElectronAvalanche]

Schematic of preamp and tremolo attached.

I introduced a 220k grid leak resistor to ground on the last triode.

I tried to at least stay somewhat close to the layout of the original amp, albeit I have the tubes mounted behind the boards, not under the boards. Attached the layout of the original. courtesy Rafael.

Electron

[ElectronAvalanche]

Here are some voltages and a head scratcher:

V1: Anode: 191V, Cathode: 3.3V
V2: Anode: 25V, Cathode: 2.2V (that is the second pentode with the 240k anode resistor)
V3a (CF): grid: 25V, Anode: 316V, Cathode: 39V
V3b: Anode: 210V, Cathode: 1.7V

PS String Voltages:

V1: 296V
V2 and V3b(CF): 318V
V3a: 323V (first in string after initial 2x47uF, 10k Series, 1k5 series, see pic).

V2 with 25V on the anode does not look right to me. Faulty tube? 240k anode resistor wrong (I remember Rootz being sceptical about this).

I need to calculate the current through V2, maybe that is also the reason for the 120hz hum (excessive current draw?).

Pic of the PS string as in Bombas schematic also attached

Electron

[ElectronAvalanche]

Looking at the AC15 schematic using EF86, the anode resistor is 220k, cathode resistor 2k2 with 25uF, so comparable, screen resistor 1M. Still in the AC15 you have 90V on the anode with 315V on the string.

https://www.ampbooks.com/mobile/classic ... /vox-ac15/

Could it be the tube? I will swap the first EF86 with the one showing the funky anode voltage and see.
Anode voltage: 25V
Supply voltage for V2 Anode: 318V
Current would currently be: 318V-25V= 293v/240k (V2 Anode resistor)= 1.22mA (anode plus screen current). So not out of the ordinary.
Cathode voltage is 2.2V
Screen voltage: 100V

Maybe the CF directly connected with ita grid to the anode is the culprit? I have no real experience with small pentodes. So maybe someone in the know can chime in.

Thank you!

Electron

[ElectronAvalanche]

Swapping the EF86 did indeed result in slightly different readings:

V2 (swapped EF86, now Telefunken, was Valvo):

Anode: 41V (was 25V with Valvo)
Screen: 120V (was 100V)
Cathode: 2.0V (was 2.2V)

V1 (now Valvo, was Telefunken):

Anode: 161V (was 191V with Telefunken)
Cathode: 3.6V (was 3.3V)

So still a riddle why the plate/anode voltage on V2 is so low. Maybe normal, maybe not, I have no idea.

Regarding the 120Hz buzz/hum:

I scoped the B+ and it is clean as can be. I scoped the DC heater voltage for the 12DW7: clean. The 6.3V (actually 5.8V) for the EF86 is quite dirty, lots of ripple, see pic.

I need to figure that out.

Best Regards!

Electron

[rootz]

Cool project! It has been some time since I’ve been tinkering with this circuit. Can’t remember the certainty of the resistor values in the schematic. Were these confirmed? Anyway, try changing that v2 screen resistor from 680k to 1meg or a bit higher. I believe the plate should be in the 90V ball park. That adjustment will get you closer.

[ElectronAvalanche]

Hi Rootz,

Thank you for your tip. I will try that. The values are not really verified of any of the components. But I take them to be correct for the time being. Maybe the Screen voltage node was different than for V2 anode in the real amp? With a lower screen voltage supply, the anode voltage would also be higher? Or maybe Dumble opted for that specific operation point.

I fixed the 120Hz hum/buzz: I swapped the LM7805 (with the two 1N4007 between the regulator and ground) with a new LM7806 and now the heater voltage is 6.0V and no more buzz/hum.

I will now play the preamp a bit and see how I like it or if I need to mod it (operation point of V2). I also consider to test whether I can insert the mentioned Vermona stand-alpne Spring Reverb unit by simply tapping into the preamp as per the amps schematic: to Reverb from Volume/Gain pot before V2 and from Reverb inserted after 3m3 resistor between CF and last triode.

Thanks,

Electron

[bepone]

PT currently is a 290V / 40mA, 14V/2A, 6.3V/1,5A. Since the PT is slighlty to tall for a 1U enclosure, this needs to be replaced by a 270V/80mA, 14V 2A unit in the near future.

The EF86s heaters are powered by a standalone voltage regulator pcb with a 7805 (two 1N4007 in series with common pin of regulator give 5,8V under load). Which is within the specs for a EF86. Maybe I will switch to a 7806 to be closer to 6.3Vs. The heater of the 12DW7 is derived from yet another voltage regulator pcb with a 7812. This regulator also powers the tremolo oscillator.

Both regulators are fed from the 14V winding of the PTand have their own FWB recitifier. Both regulators are grounded via the 78s directly to chassis (acting as a heat sink as well).

I get a faint 100Hz hum (independent on preamp gain). In my previous HRM preamp I also had this due to too little filtering of the regulator (fixed by going up to 2200uF on the regulator input and 1000uF on the output side.

I wonder if somebody might have an opinion on the 100hz noise. Could it be that having two FWB from the same 14V widing is producing the ripple? I could of course take the unregulated rectified voltage of one of the pcbs to feed the second regulator pcb . The pcbs allow this via dedicated unregulated outputs. Or should I lift the 12V regulator from the chassis heat sink?

no problem all easy to fix ill check the text and scheme today. for the start lift 6V regulator from chassis and leave it in the air, power on shortly( because you will have overheating in this regulator- this you need to take care- modify can be too much heat developed to take it from the 14VAC)and monitor the output - +6VDC to the heater - if is ok

[ElectronAvalanche]

Hi Bepone,

Thank you for your reply. I managed to fix the 100hz noise. It was ripple from the 6.3V DC supply for the EF86s. I used a 7805 with two diodes between the 7805 ground and circuit ground to boost the voltage to 5.7V and that posed some problems. I changed the 7805 to a 7806 (without diodes) and now have a clean 6.0V supply and no more 100hz hum (I also lowered the second filter cap on the output side of the 780x to 470uF (was 1000uF and maybe too much).

I slso tried to passively take the output signal for an external Spring Reverb unit from the Gain pot (as per the original schematic) and return to after the 3m3, but that is not yet working really, might need a series resistor (220k) on the send. But the main problem with that is noise. More experimenting.

So if I keep the preamo without passive Reverb intersection as mentioned above, I might reduce the 3m3 resistor a bit to get a hotter output from the preamp and maybe also to get some more clarity. Right now the preamp is on the darker side and I need to habe the bright switch (150pF) enganged.

Electron

[bepone]

Hi Bepone,

Thank you for your reply. I managed to fix the 100hz noise. It was ripple from the 6.3V DC supply for the EF86s. I used a 7805 with two diodes between the 7805 ground and circuit ground to boost the voltage to 5.7V and that posed some problems. I changed the 7805 to a 7806 (without diodes) and now have a clean 6.0V supply and no more 100hz hum (I also lowered the second filter cap on the output side of the 780x to 470uF (was 1000uF and maybe too much).

this was my point too ok

[bepone]

So if I keep the preamo without passive Reverb intersection as mentioned above, I might reduce the 3m3 resistor a bit to get a hotter output from the preamp and maybe also to get some more clarity. Right now the preamp is on the darker side and I need to habe the bright switch (150pF) enganged.

Electron

3m3 is taking a lot of heighs, so try to put 10pF or 20pF in parallel with it, this will inrease heighs, you can put cheap ceramic cap there

[ElectronAvalanche]

So again on the V2 low plate voltage topic:

I bypassed the 0.1 screen capacitor (Cs in pic below) with either a 1M, 820k or 680k resistor and this brought the plate voltage up and the screen voltage down. In case of the 680k I had about 85V on the plate and 83V on the screen.

But the sound changed quite a bit: the original low plate (45V) and high screen voltage curiously sounds best: nicely compressed, even frequency response. Higher plate voltages and lower screens resulted in peakier sound, more high mids, a bi t more bass. All in all I liked the original setup better.

Of course there might be a difference in changing the screen resistor from 680k to 1M compared to my „resistor in parallel with screen cap to ground“, so maybe I will swap the 680k for a 1M in a next experiment.

Electron

[ElectronAvalanche]

Update:

I now have a 18pF across the 3M3 and this gives some nice clarity (not that the sound was muffeled before, just a bit more sparkel now).

I am still wondering about the Local Negative Feedback around V2. Max ("just the facts") and somebody called Jay stated many years ago, that the LNFB on V2 was not between plate of the pentode and the grid, but between the grid of V2 "and the cathode of a third tube". So clearly they must mean the Cathode of the CF after V2 (the voltages of V3b (triode) of V3 would be too small to have any effect with 40M in the feedback loop).

So I wonder if this is a smoke screen or if there really is something to it. I guess I need to try it. But before, maybe someboday has an opinion about the feasibility of such LNFB between cathode of V3a (12AU7 part of the 12DW7) and grid of V2.

The phase of the signal on V3a would be the same as of the plate of V2? There would be no additional gain by the CF (V3a). Voltage swing would be higher? Does anyboday have experience with such a LNFB in such a setting: pentode into CF?

Maybe this will start a discussion. I might simply detach the current LNFB and rig up a new one between cathode of V3a and grid of V2. Let's see if I have enough 10M in my stash to do this.

And one more thing: the Tremolo of course robs some signal when engaged (LDR connected to in of Master volume). I might lift the Master Volumen via a 3PDT switch by some set resistor to compensate when switching in Tremolo. Difference is a 1/4 turn of the Master (1M) to set equal levels.

Electron

[ElectronAvalanche]

So final posting on this preamp:

So the questions about the LNFB is somewhat solved for me. I tried the LNFB from either V2 plate to V2 grid (as in the schematic by Bombacaototal) or from the cathode of the CF after V2. Almost no real difference. I recorded the two different LNFB variants using a looped guitar signal and then overlayed the audio in Logic and switched the phase in one of the audio files to hear any difference. The V3a LNFB (from the CF cathode) was minutely brighter, but really almost not discernable.

So I used the EQ Match function of Logic to try to set have Logic show the EQ difference between the two setups. Slight (2-3db) boost in highs with the V3a LNFB as compared to the V2 plate LNFB.

The LNFB really does have an impact on the sound if you leave it out completely. To visualize this I had Logic try to match the EQ between no LNFB and either V3a LNFB and V2 LNFB. Here is a pic of that attached. The upper graph shows the EQ needed to achieve the sound without LNFB for LNFB from V2 plate to V2 grid, the lower from V3a cathode to V2 grid.

What you see is that LNFB reduces the bass quite a bit (a bit more pronounced in the below 50Hz for V3a LNFB) and adds some high frequency content. Again there is no marked difference between LNFB from V2 plate to V2 grid or in case of V3a Cathode to V2 grid. So the comments by Max/jay might be right, that Dumble chose to have LNFB from V3a cathode rather than V2 plate to V2 grid, but the difference is really so minute that it does not matter in my opinion. Using a different speaker cab would make an extreme difference, maybe even fluctuations in ones preceived hearing on a given day or mabye line voltage?

Who knows? I for now kept the V3a cathode to V2 grid LNFB setup to be more in line with the "gospel". Next up is the installment of the final power transformer, setting the voltages again and putting on lettering for the coontrols.

In summary:

Bombacaototal's schematic works and results in a nice sounding, chimey, clean to slightly overdriven Blues amp. If you want to get closer to "the facts" use the V3a to V2 grid LNFB. All in all time worthwhile spent on making this preamp. The beauty of building dedicated preamps is the fast build time, easy way of combining the preamps with different power amps (I can e.g. use the preamp either with a Marshall-style 2xEL34 power section or with the 4x6L6 SSS 002 (CF) power section at will. Totally different sounds, yet simply different colour palettes.

I guess Dumble also had this envisioned with his Phoenix preamp and I seem to remember he said in an interview that almost all his designs could (!) be available in such preamp/power amp form.

Enjoy building and thanks to Bombacaototal, Tony, Aaron and everybody else for the schematic.

Nice sound samples will follow, promised.

Electron

[Bombacaototal]

Awesome thread Electron, so happy to see that you build a preamp of the Alligator and thanks for documenting everything here and reporting your findings

This is definitely my next amp, when time is right and money is less tight!