Hoffman stereo pre-amp

[Deude_Canned]

Hello all, great web site and forum. I found this site by way of DIYstompboxes.

Long story short I want to build a stereo preamp for a bass guitar, and I believe the Hoffman stereo preamp is just what I need. My bass is wired for stereo output (EMG Geezer Butler P-J on one channel, bridge piezo on the other) and I want to process these signals separately. I am not using a mic on an amp but direct studio style recording. Right now I am using a small non rack mount mixer to preamp the bass, and a separate box for overdrive. If I can get it all together that would be great.

Here is the design page
https://el34world.com/projects/StereoPreAmp1.htm

I like the fact that this design is driven at full voltage and not a starved valve design; it is going to be rack mounted so high voltage and more power is OK with me and worth it for the improved sound. I have read through the build instructions and believe I can handle it as I have built similar things before, but I am new to tube amps.

I have a few questions before I start the build...
1. Can the stages be driven to overdrive with the gain setting? This is something I want to be able to do on one of the two channels.
2. Is this design robust enough to play around with different tubes? I have not found it in the details yet but I presume this uses the usual 12AX7 tubes, and I could try out a 12AU7 for example without changing the circuitry.
3. It looks like there is no board available for this since Hoffman mentions it is not a kit, but a one-off build. Is there a place where I can get the board for this? If not then I can just build it on phenolic perfboard or the like.
4. Is this amp quiet? Any significant problems with hum?
5. I see some twisted wires in the pictures of the completed board, like the green wires between the tube mounts. I presume that is for noise rejection a-la common mode. Is that correct?
6. Since Mr. Hoffman is out of the parts business (?) what source is generally recommended for parts?

Thanks in advance

[Deude_Canned]

One more question... Has anyone put together a parts list for this? I am sure I can build one but if one already exists it would help.

[EL34]

Hello all, great web site and forum. I found this site by way of DIYstompboxes.

Long story short I want to build a stereo preamp for a bass guitar, and I believe the Hoffman stereo preamp is just what I need. My bass is wired for stereo output (EMG Geezer Butler P-J on one channel, bridge piezo on the other) and I want to process these signals separately. I am not using a mic on an amp but direct studio style recording. Right now I am using a small non rack mount mixer to preamp the bass, and a separate box for overdrive. If I can get it all together that would be great.

Here is the design page
https://el34world.com/projects/StereoPreAmp1.htm

I like the fact that this design is driven at full voltage and not a starved valve design; it is going to be rack mounted so high voltage and more power is OK with me and worth it for the improved sound. I have read through the build instructions and believe I can handle it as I have built similar things before, but I am new to tube amps.

I have a few questions before I start the build...
1. Can the stages be driven to overdrive with the gain setting? This is something I want to be able to do on one of the two channels.
2. Is this design robust enough to play around with different tubes? I have not found it in the details yet but I presume this uses the usual 12AX7 tubes, and I could try out a 12AU7 for example without changing the circuitry.
3. It looks like there is no board available for this since Hoffman mentions it is not a kit, but a one-off build. Is there a place where I can get the board for this? If not then I can just build it on phenolic perfboard or the like.
4. Is this amp quiet? Any significant problems with hum?
5. I see some twisted wires in the pictures of the completed board, like the green wires between the tube mounts. I presume that is for noise rejection a-la common mode. Is that correct?
6. Since Mr. Hoffman is out of the parts business (?) what source is generally recommended for parts?

Thanks in advance

I am not out of the parts business
I am not building amps or doing repairs


The guys here can help with your questions

[shooter]

1 sure, just change some of the voltage dividers and 2 AX7 WILL overdrive
2 I just plug in a tube til I get one I like, so ya, you can change tubes (assuming the same pinout, N family resemblance)
3 look in other topics for DIY layout creator
4 If built correctly
5 filament wires, see #4
6 Doug, or surf up electronic parts for sale

[SILVERGUN]

You'll notice that both channels are exactly the same and this was obviously done on purpose.
If I were making a bass preamp I would make the channels sound different and then blend the end result to get the sound I was looking for.
You will probably want the tonestack to be less mid scooped and you wont need this much gain, especially with active pickups.
There are ways to alter this design to reduce noise and reducing overall gain is one way.

[Deude_Canned]

Thank you Shooter. From what I read above Doug Hoffman is out of the parts business so I need another source, right? I can find all of these parts from various sources but it would be nice if there was a one-stop shop (or two-stops) for expertise and shipping cost savings. We used to have a great analog components shop locally that had EVERYTHING but they went out of business.

Silvergun good suggestion on having the two channels sound different. Actually the EMGs I am using are passive and are both on one channel. They have separate volume knobs and a common tone control. The bridge piezo is on the other channel (with its own volume knob) and inherently sounds radically different from the EMG signal. BTW this is all in an Ibanez bass that I modified and rewired. 

The piezo is extremely bright of course compared to the EMGs and is all upper end sound; it sounds more like an acoustic steel string guitar than a bass. What I am doing is running the piezo clean for attack, and then overdriving the EMG signal so I get the growl but the overall sound is still clear. It's akin to the way Chris Squire (RIP) ran his bass setup; IIRC he played a two-channel Rickenbacker using a seperate amp for each channel, running overdrive on one of the two. I love his sound, timeless.

I pan one channel slightly left and the other slightly right in the final mix and it gives the sound a lot of depth (spread) that way, kind of like a panned, short delay.

[Deude_Canned]

So I have not seen these particular soldering post components before. It looks like they through-bolt to a phenolic board to create a nice point to solder component leads onto, and act as a through board via. Can anyone tell me what these are called so I can procure some?

[66Strat]

Thank you Shooter. From what I read above Doug Hoffman is out of the parts business so I need another source, right? I can find all of these parts from various sources but it would be nice if there was a one-stop shop (or two-stops) for expertise and shipping cost savings. We used to have a great analog components shop locally that had EVERYTHING but they went out of business.

Silvergun good suggestion on having the two channels sound different. Actually the EMGs I am using are passive and are both on one channel. They have separate volume knobs and a common tone control. The bridge piezo is on the other channel (with its own volume knob) and inherently sounds radically different from the EMG signal. BTW this is all in an Ibanez bass that I modified and rewired. 

The piezo is extremely bright of course compared to the EMGs and is all upper end sound; it sounds more like an acoustic steel string guitar than a bass. What I am doing is running the piezo clean for attack, and then overdriving the EMG signal so I get the growl but the overall sound is still clear. It's akin to the way Chris Squire (RIP) ran his bass setup; IIRC he played a two-channel Rickenbacker using a seperate amp for each channel, running overdrive on one of the two. I love his sound, timeless.

I pan one channel slightly left and the other slightly right in the final mix and it gives the sound a lot of depth (spread) that way, kind of like a panned, short delay.

No. Doug is still selling parts, as he previously indicated. His on-line store is on this website. Scroll down to the bottom of the page, or click on this link.
https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&thispage=partsenter.htm&ORDER_ID=!ORDERID!

So I have not seen these particular soldering post components before. It looks like they through-bolt to a phenolic board to create a nice point to solder component leads onto, and act as a through board via. Can anyone tell me what these are called so I can procure some?

Turrets. Doug sells complete turret boards or the individual components. Check out the catalog page that I linked.

[Deude_Canned]

Thanks I have found his store and started building an order.

Doug mentioned I should talk to Mojo about the parts for the power supply section, and the preamp tubes, since he does not carry them. A couple of the part numbers don't exactly match the Mojo online catalog so I called them for help:
- Mojo 779 transformer, no problem
- Diode bridge, they did not have them but I can build one easy enough if I can figure out the right diodes.
- The problem was with the choke. I gave them Doug's part number (125C3A) and the person I spoke with asked what tube I was using in the power section. I had an 'uhhhh' moment because I do not see one on the schematic. All I see is the small circuit for the heater power supply.
https://el34world.com/projects/images/stereopreampschematic.gif.

The person I spoke with was of the strong opinion there needs to be a tube in there somewhere, but even looking at Doug Hoffman's build pictures I do not see one in the power supply section.
https://el34world.com/projects/images/DSCN1883.JPG

So what is going on here? AM I missing something?

[shooter]

Fender Style Chokes 
Part#  Current Henry Mounting Height Length Width Weight
125C3a  50ma 4 2"  1 3/8"  1 11/16"  1 3/8"  6 oz 
125C1a  90ma 4 2 13/16"  2"  2 5/16"  1 3/4"  15 oz 
from Dougs notes on chokes;
so;
decide how much current you'll be drawing, surf up "choke 4Henries, 90mA" and click away

[Deude_Canned]

OK so no power supply tube required then? Thanks for the choke info.

[shooter]

So what is going on here? AM I missing something?

It's your amp, your call, want a tube rectifier, use one, want 1/2 wave bridge diode, done, want a full wave.......
you probably should start with your vision full schematic, then it's way easier to say; "ya, the coupler needs to be......"

[Deude_Canned]

Basically I want to copy Hoffman's design because it is known to work, and then mod from there. I am OK with using a SS diode rectifier bridge, no problem there. Looking at the picture of the one Hofman built there is a small box in the power supply that looks to me like it could be a SS diode rectifier bridge. I am seeing now they can be bought as a package and that would make life easier. I'd be happy to buy one and go.

[j_bruce]

https://www.amplifiedparts.com/products/bridge-rectifier-single-phase-3a-400v

[sluckey]

Basically I want to copy Hoffman's design because it is known to work, and then mod from there. I am OK with using a SS diode rectifier bridge, no problem there. Looking at the picture of the one Hofman built there is a small box in the power supply that looks to me like it could be a SS diode rectifier bridge. I am seeing now they can be bought as a package and that would make life easier. I'd be happy to buy one and go.

Hoffman sells that same bridge.

[EL34]

Basically I want to copy Hoffman's design because it is known to work, and then mod from there. I am OK with using a SS diode rectifier bridge, no problem there. Looking at the picture of the one Hofman built there is a small box in the power supply that looks to me like it could be a SS diode rectifier bridge. I am seeing now they can be bought as a package and that would make life easier. I'd be happy to buy one and go.

You are not spending enough time browsing the store to gather your parts

If you have a paper catalog in front of you, just turn the pages and look to see what is in the catalog
Same with a web store like mine
A diode bridge is easily found here
https://hoffmanamps.com/MyStore/catalog/Lamps.htm

Discontinued parts in my store all have links that lead you to a page here on the forum with info on where to get those parts

[Deude_Canned]

Thanks I found the diode bridges yesterday and have them in my cart already. I have also been scouring the catalog quite a bit, and even found the Mojo 779 transformer that you have on closeout. Are these actually not in stock since you referred me to Mojo?
https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Transformers&ORDER_ID=!ORDERID!

To be clear, my confusion was not with the diode bridge itself, just the fact that the person at Mojo told me there must be a tube in the power supply section. He must have assumed I was not going to use a SS diode bridge. I do not want to use a tube in the supply, I never did, a SS diode bridge is fine with me, and I found it in Hoffman's store. It's in my cart. 

[sluckey]

To be clear, my confusion was not with the diode bridge itself, just the fact that the person at Mojo told me there must be a tube in the power supply section.

Mojoman didn't know what he was talking about.

[Deude_Canned]

After he told me that, I looked at the schematic again and verifying I definitely did not see a tube in the power supply section, I read more in the forums and saw that some build rectifiers with tubes, so I can only presume that is the only way that person at Mojo knows to how to build amp power supplies.

Nothing against Mojo... The guy I spoke with was in sales and not in tech support. 

[EL34]

Thanks I found the diode bridges yesterday and have them in my cart already. I have also been scouring the catalog quite a bit, and even found the Mojo 779 transformer that you have on closeout. Are these actually not in stock since you referred me to Mojo?

Yes, I have one more Mojo779 left in stock and then they are all gone
It is here on this page
https://hoffmanamps.com/MyStore/catalog/Transformers.htm


Items that I already discontinued are listed here.
https://el34world.com/Forum/index.php?topic=23827