Hammond H1 conversion(UPDATE: BIAS QUESTION REPLY #37)

[jordan86]

I’ve now got the Hammond chassis. Threw it in the car since I’m hitting the road for a quick trip. Im headed to see a fellow amp tinkerer tonight. We are gonna try to get it working tomorrow. A few questions...

1) Is possible to clip into to the amp, say from just after the phase inverter of an AB763, or AC15, or maybe a 5e7 without hurting the first amp? Would it be best to pull the power tubes on the “donor” preamp?

2) We may drill the chassis to get preamp tubes installed. I HIGHLY doubt my buddy has any 6 volt preamp tubes. Is it problematic to try a 12ax7? I’ve never wired heaters with anything other than 12ax types. I don’t think it’s a problem to mix and match though.

[shooter]

keep it simple, provide audio source to your PA from a phone, an mp3, a sig-gen. leave the good amps good.

[sluckey]

1) Is possible to clip into to the amp, say from just after the phase inverter of an AB763, or AC15, or maybe a 5e7 without hurting the first amp? Would it be best to pull the power tubes on the “donor” preamp?

Yes. Just pick up the two signals after the coupling caps. If the amp will have negative bias on the grids, you need to use a cap to block that. Yes, pull the power tubes.

2) We may drill the chassis to get preamp tubes installed. I HIGHLY doubt my buddy has any 6 volt preamp tubes. Is it problematic to try a 12ax7? I’ve never wired heaters with anything other than 12ax types. I don’t think it’s a problem to mix and match though.

12AX7 is typically connected to 6.3v filaments. Tie pins 4 and 5 together and connect filament wires to pin 9 and pin 4/5.

[jordan86]

Just thought I’d share. Got her functional last night. Repaired a few holes and tears in one speaker, wired both speakers in, and got the mains wiring functional. Fired right up. Amazing. Working on a preamp today. I’d like to toot my own horn with my mega ghetto (and temporary) mains wiring. :)

[sluckey]

Exciting isn't it? Don't forget, you will need a preamp and a PI. I'd do something simple with two tubes. Lot's of choices.

Oh yeah, I like your chassis cradle.

[jordan86]

Yes, we are pouring over a few shcematics with simple stage 1 and PI scenarios. 18 watt and the Carmen Ghia are the front runners at the moment.

So is isolating the speaker voice coil tap an option?

[sluckey]

So is isolating the speaker voice coil tap an option?

Not sure what you mean by that???

[jordan86]

So is isolating the speaker voice coil tap an option?

Not sure what you mean by that???

Can I connect just the field coils on the stock speakers but NOT the voice coils. Would send the speaker outs to a different working cab with PM speakers.

[sluckey]

Can I connect just the field coils on the stock speakers but NOT the voice coils. Would send the speaker outs to a different working cab with PM speakers.

Yes, you can do that.

[jordan86]

Haven’t made any progress on the H1 but I plan to order parts and work on it later this month. We hit a roadblock. Got the speakers and 5e7 preamp connected through some additional coupling caps. Worked great for about 8 seconds. Then started cutting out and doing a farty buzzing thing. Like a fuzz pedal with the bias set wrong. We did some QUICK voltage checks. I think it’s a cap down line. Maybe the coupling caps coming off the 6SN7. Voltages seemed good at the cap can, but quite low on the 6sn7. So I’ll tackle that.

My main question though is about a tonestack loss. I know a lot of one knob tone controls are touted low loss, and the standard TMB ala Fender is very lossy. In the spirit of Dr Z/Ghia, I’m looking at that tone stack. Used on both the Ghia/Pheonix as well as the Stingray by Dr Z. I like the idea of the Stringray stack attached. My questions are:

1) Would this be low loss or medium loss?
2) How might one tell?
3) Any practical explanation on how differently the Stangray values would sound from the Phoenix?

At this point I am thinking parallel 12ax7 triodes at V1 into the attached stingray tonestack and PI, into the H1 power amp.

[PharmRock]

You and I are looking at the exact same tone circuits for different applications.  See this thread: https://el34world.com/Forum/index.php?topic=28900.0

I really like the Carmen Ghia tone control.  Its more of a mid-sweep control with sort of "wah-like" properties.  I believe Dr. Z lifted this from the mid-pot control of the Matchless Chieftain, with modifications. https://el34world.com/charts/Schematics/files/Matchless/Matchless_chieftan.pdf

It seems most useful between 10 and 2 o'clock.  Hope you can get it to work for your amp; looking forward to seeing how you implement this. 

[tubeswell]

In the spirit of Dr Z/Ghia, I’m looking at that tone stack. Used on both the Ghia/Pheonix as well as the Stingray by Dr Z. I like the idea of the Stringray stack attached. My questions are:

1) Would this be low loss or medium loss?
2) How might one tell?
3) Any practical explanation on how differently the Stangray values would sound from the Phoenix?

To start with, in the schematic you posted V1 cathode only has a 0.68uF Ck, so is not fully bypassed - it has a 1/2-boost roll-off just above 109Hz (which is the 5th string on your guitar). Below that frequency, Voltage gain (A) is only 10. Above that A = 38. So it rolls off the lowest 5 frets on the bottom E string, which you might or might not like.

For those unbypassed frequencies, output impedance at the plate of V1 is 77k, and the load (comprised of the tone stack) is 61k (with all the controls set to 'maximum signal'), so the loss is 61k/(77k + 61k) = 44% of the gain at the plate - at best.

For the bypassed frequencies, output impedance at the plate is 38.5k, so 61k/(38.5k + 61k) = 61% of the gain at the plate = better


You will get some improvement if you have an extra triode with which to parallel v1, depending on whether your parallel stage is set up for same gain-more current, or more gain-slightly less current. Or you could use the extra triode for a CF buffer to drive the tone control.

The tone control is a band-stop, bridged T-filter with an adjustable mid-scoop-kinda-tilt control.  You build what you like

[shooter]

^^^^what he said

1) Would this be low loss or medium loss?2) How might one tell?

OR;
use a scope.  I measure the input signal against the output of the TS.
I use 3 frequencies, ~80hz, 880hz, and 4K for guitar amps.  I measure with TS knobs at full ccw, 1/2 way, and full cw

[jordan86]

For the bypassed frequencies, output impedance at the plate is 38.5k, so 61k/(38.5k + 61k) = 61% of the gain at the plate = better

Super helpful Tubeswell, as usual. I am going to plan on doing something more in the 10-25uf bypass on V1 for a more fully bypassed first stage. Do you have a ballpark percentages of what the loss is on say a tweed or 18 watt single tone control, or on the AB763 TMB stack? Just so I could compare that?

You will get some improvement if you have an extra triode with which to parallel v1, depending on whether your parallel stage is set up for same gain-more current, or more gain-slightly less current.

Since I have two tubes, and a low need for dirt, I am planning to use V2 for a LTPI, and  V1a and V1b in parallel for a first stage. Practically speaking, I'm not sure how to accomplish the more gain vs more current thing, or what the final outcome would be tonewise. Are you talking like how hot the cathode is biased (say 820R vs 2.7K) and plate load resistor values? Where could I learn more about that, and maybe see some example values?

The tone control is a band-stop, bridged T-filter with an adjustable mid-scoop-kinda-tilt control.  You build what you like

I'm not sure what I like, I'm just trying to do something "simple-ish" that's not something I already have, and that might fit this chassis. I have a single ended Super Lead sorta thing (CF>TMB), an EF86 into EL84 Dr Z(B+T), another EL84 dr z(TMB), a 65 Dual Showman, and a Princeton Reverb.   

It really is a toss up between this Dr Z tone stack and the 18W. I don't think I want to go tweed with this chassis, but maybe an 18W. My plan was to do something cleaner, but still unique. If clean can even be a unique thing 

[jordan86]

One other question, regarding converting this to an adjustable fixed bias...would it be best to replace R9 with a 50K-100K pot? Or R10 with a 25K? Or something else?

[sluckey]

I'd try something like this...

[jordan86]

The 100 ohm resistors for R11 and R12 on the schematic are not present in my chassis. (Photo attached) Just a plain green wire connecting Pins 5 on the 6V6s. Should that be concerning?

Most of the resistors are pretty far out of spec too. 15-20%. Should I be relentless toward replacing all those before proceeding with a preamp?

R1:3310 (3900)
R2: 3270 (3900)
R3: 3550 (4700)
R4: 77.7k (68k)
R5: 72.7k (68k)
R6: 67.3k (68k)
R7: 73.2k (68k)

[tubeswell]

Most of the resistors are pretty far out of spec too. 15-20%. Should I be relentless toward replacing all those before proceeding with a preamp?

R1:3310 (3900)
R2: 3270 (3900)
R3: 3550 (4700)
R4: 77.7k (68k)
R5: 72.7k (68k)
R6: 67.3k (68k)
R7: 73.2k (68k)

Those look okay to me for rock'n'roll. I wouldn't bother changing them unless you find its inordinately hummy once you plug it in (which it may or may not be - i.e. don't just go changing them out for the sake of it yet).

[jordan86]

Here is my ugly copy/paste attempt at adding a preamp. I really need to learn a better program to do this well. Would love your guys feedback on ensure this works before I run with it. Open to tweaks from a tonal standpoint as well. 

Its just a standard 68K/1 Meg singe input > parallel 12Ax7 stage > Dr Z Stangray tone stack and LTPI > Hammond power amp

A few things I am most curious about...

1) Can I run both preamp tubes off the 'D node' I created? I'm not sure why the big R8 6K/10W on the hammond. I believe its to daisy chain power amps together.  But I used it as a dropping resistor for the preamp filtering. I may need to change it out though with a different value.

2) Can I run the LTPI straight into the 6SN7 grids through two coupling caps like I did here? Not sure if I need grid stoppers or voltage dividers, etc. I assume it will "work" without them, but I guess I won't really be able to assess the tone till I try it.

3) FWIW...I'm not sure how I want to bias the parallel stage V1. That may change as I dig into it more. I know many people do it differently. Used that layout moreso for the ease of copy/paste.

4) Lastly, I'm not sure if C8 on the LTPI is necessary?

[sluckey]

1) Can I run both preamp tubes off the 'D node' I created? I'm not sure why the big R8 6K/10W on the hammond. I believe its to daisy chain power amps together.  But I used it as a dropping resistor for the preamp filtering. I may need to change it out though with a different value.

The preamp is usually run from a different PS node from the PI. I suggest you use another 1K resistor and 22µF cap to make another node. see power supply in http://sluckeyamps.com/dual_lite/dual_lite.pdf

2) Can I run the LTPI straight into the 6SN7 grids through two coupling caps like I did here? Not sure if I need grid stoppers or voltage dividers, etc. I assume it will "work" without them, but I guess I won't really be able to assess the tone till I try it.

No. You must have a resistor connected from grid to ground on each triode. Typical value is 1M. see V2A in http://sluckeyamps.com/rocky/rocky.pdf

3) FWIW...I'm not sure how I want to bias the parallel stage V1. That may change as I dig into it more. I know many people do it differently. Used that layout moreso for the ease of copy/paste.

I used 100K plate and 820Ω cathode with good results. see V1 in http://sluckeyamps.com/dual_lite/dual_lite.pdf

4) Lastly, I'm not sure if C8 on the LTPI is necessary?

Yes, it's necessary. It's more common to see a .1µF cap for C8.

[jordan86]

Thanks Sluckey!!!

[jordan86]

Just noticing that this circuit has no screen grid resistors on the 6V6’s?!? Appears that the screens are connected to the OT primary center tap? Logic tells me that’s by design and not a problem. Is this because of the field coil speaker design? I’m only accustomed to seeing screens connected to the second filter node then through high wattage resistors.

Would it still be advisable to add screen grid resistors or would that be unnecessary based on the field coil speakers?

[sluckey]

I thought you were gonna leave the power amp as is?

[jordan86]

I thought you were gonna leave the power amp as is?

I am planning to. I just don’t understand why there are no screen grid resistors.

[PRR]

> don’t understand why there are no screen grid resistors.

Because, unlike Fender, Hammond didn't over-volt his amps.

To be fair, Fender expected his amps to move 6 nights a week, Hammond expected to stay in the same church for a lifetime, so hot-rodding was not called for.

[jordan86]

So I have finally wired this up. Please be aware, I am an enthusiastic amateur. This is not only my first go at converting an amp, but my first attempt at building a preamp from scratch (without a layout), as well as my first attempt at flying parts point to point like this.

Ive attached pictures though I don’t see how they will be helpful.  I believe my wiring is correct but obviously need to verify again.

The speakers make VERY little to no noise. There is a fairly loud hum/buzz coming from the chassis itself. Like there’s a lot of current going somewhere, just not to the speakers.  Can feel the chassis itself pulsing.

- Preamp voltages seem to check out. I have 220v on the plates of the PI. 150v on the stage 1 plate and 1.47v on the cathode.
- Most amp voltages check out as well from the Hammond schematic.

Any ideas on where to start troubleshooting? I don’t have a scope. Only a multimeter.

[jordan86]

Here's a video of the sound. The low hum is constant. The higher squealing comes in an out as I adjust the Vol/Tone controls.

Note: This sound is not coming out of the speakers. It's coming from the chassis itself. 

[shooter]

now, ground the input to chassis and retest.  NO cords, gator-clip tip to chassis


the 1st sound as volume went up sounds like a parasitic oscillation, easily seen, typically viewed at plate

[jordan86]

now, ground the input to chassis and retest.  NO cords, gator-clip tip to chassis


the 1st sound as volume went up sounds like a parasitic oscillation, easily seen, typically viewed at plate

I get no squeal sound with the input grounded. Except for the loud constant him still.

I did not observe any oscillation but I believe it may be in the 6SN7. I can feel that tube physically vibrating when sound happens.

[jordan86]

The only change from this schematic I made is 1M resistors to ground on pins 1&4 of the 6SN7, per Sluckeys recommendation.

I’m wondering if this poweramp doesn’t need a PI at all?!?

[shooter]

in order for a traditional PP PA to work the PA needs an "in phase" and an "inverted" signal, so one side gets a signal "in-phase" with the input, the other "side" gets a flipped - inverted signal. 

Except for the loud constant him still.

try swapping the plate wires at the PI

If the hum is still there (no signal in), scope it out at the speaker, if you can hear it, you can see it. 
IF there's NO sound at the speaker, then swap out the tube making the sound.

[jordan86]

No luck swapping the leads coming off the PI plates. I even tried jumpers off the PI over the 6sn7, straight to the grids of the 6V6. Nada. Also tried bypassing the tonestack. No luck there either.

I’m thinking this power amp circuit is just too atypical for me to get working. Goodwill is moving closer toward the top of the next to try list 

Does that video clip sound like a bad component? Wondering if I could have unintentionally cooked the silver mica cap in the tone stack.

I don’t have any 6sn7 tubes to swap. I do have a local tech with a scope that I could ask to look at it. Paying would be insult to injury.

[jordan86]

Just got some more intel from a Hammond guy. This is somewhat of a duplicate post, as I now have two threads converging.

The first stage 6SN7 on my poweramp was typically fed from a Hammond AO-28 or similar chassis. Which appears to have an OT driven by a 12BH7. See schematic.

http://captain-foldback.com/Hammond_sub/schematics/ao28.jpg

This is way beyond my pay grade. Not sure at all now how I can build a preamp into the H1 poweramp, or if my idea even has potential.

[sluckey]

I replied to your other thread.

[jordan86]

Well...time for the post of shame.


I fired up the amp once more and looked at the schematic to make sure there wasn’t just some stupid glaring error on my part. Well, there was. There are two screws drilled through the chassis for the speakers. I thought they were both for ground and was using them interchangeably. Turns out one is just to parallel the voice coils and it needs to be isolated from the chassis. Due to my oversight, my voice coils were just grounding out to the chassis. Doh!!!

Sounds loud and clear now. Still a bit noisy and humming but I think I’m in a good place now. Thanks all for your patience!!!

[TemChamp]

Hello All,
This conversion is fantastic! I have one of these in my office that I'm trying to work out. Did you make the amp work with the schematic posted previously, or were there any other changes made to that? If so, do you have an updated version?

Thanks!

[jordan86]

I take no credit. My buddy supplied the chassis. Had 4 of these sitting in a storage unit. So many here were amazingly helpful with all my stupid questions and curiosities. I did get it “working”. Don’t have a proper evaluation on it yet, as I’ve not loaded the speakers into a cabinet since I got it working.

I also have zero skills in building a schematic on the appropriate software. I’d be open to pay someone a few bucks though to try that.

[jordan86]

Now that I have this working....I'm stumped on how to check the bias   

As a newbie, I've learned the typical ways to check bias for cathode and fixed bias amps (cathode resistor, OT voltage drop, 1 ohm resistor on cathode). But this whacky vintage organ amp has me stumped. Any idea how I might take measurements and figure out the bias on the power tubes or output transformer?

The original 6v6's still work and sound good. I'm curious about running a pair of bigger bottles vs the stock 4x6v6. It sounds incredible with two 6L6s but I have no clue how to check the bias.

[PRR]

Add 1r resistors, or spend for a BiasRite.

[jordan86]

Add 1r resistors, or spend for a BiasRite.

It may be time for some bias rites. I’ve held off for too long :)

Would I add the 1r at the cathode? Typically the cathode is grounded in a fixed bias amp but this one is different. This OT with multiple secondaries is not something I’ve ever seen before. Has me stumped.

[sluckey]

Disconnect whatever is connected to pin 8 of a 6V6. Now connect one end of a 1Ω/1W/1% resistor to pin 8 of that socket. And connect whatever was disconnected to pin 8 to the dangling end of that 1Ω resistor. Repeat for the other tubes.

To measure current for a tube connect your meter (which is set to measure mV) ACROSS the 1Ω resistor.