1.5k resistors for 4 power tubes

[sjwood3]

This may seem simple to some of the experts.  I'm building a Hoffman Twin Reverb with 4 power tubes instead of two.  No rectifier.  The picture I've included is of the Ab763 two channel power tube section.  I understand how to jumper the two additional tubes into the mix, but do I need the 1.5k resistor in the same position on the V8 and V10 power tubes?   

[sluckey]

do I need the 1.5k resistor in the same position on the V8 and V10 power tubes?

Yes. Then just connect pin 6 of the two tubes together.

[sjwood3]

To clarify, jumper pins 6 on v7 to v8 , then separately jumper pin 6 on v9 and v10?  This is assuming I'm following the Hoffman advise on jumpering the two new tubes into the system.

[sluckey]

That's correct. But be careful. It's easy to get mixed up with this.

[sjwood3]

Of course.  I really appreciate your feedback.   Check and double check!

[tubeswell]

Same thing applies for each 3W 470R screen grid resistor going to pin 4 (between Pin 1 and Pin 4). The opposite pairs can be linked by a single wire between Pin 1 on each pair, with only one of each pair needing another wire going to the screen supply node on the main board.

[sluckey]

He's following Hoffman's layout... All screen resistors are on the board and 1.5K stoppers are mounted between pins 5 and 6. And pin 1 is jumpered to pin 8. I fear the Fender stuff may just add a bit of confusion.

[2deaf]

He's following Hoffman's layout... All screen resistors are on the board and 1.5K stoppers are mounted between pins 5 and 6. And pin 1 is jumpered to pin 8. I fear the Fender stuff may just add a bit of confusion.

Sorry.  I'll remove it.

[sjwood3]

Same thing applies for each 3W 470R screen grid resistor going to pin 4 (between Pin 1 and Pin 4). The opposite pairs can be linked by a single wire between Pin 1 on each pair, with only one of each pair needing another wire going to the screen supply node on the main board.

So is this alluding to a fender layout as well or the Hoffman layout?

[tubeswell]

Same thing applies for each 3W 470R screen grid resistor going to pin 4 (between Pin 1 and Pin 4). The opposite pairs can be linked by a single wire between Pin 1 on each pair, with only one of each pair needing another wire going to the screen supply node on the main board.

So is this alluding to a fender layout as well or the Hoffman layout?

The red circled bit (attached) - from the Hoffman layout you posted earlier

[sjwood3]

I see.  I'm not using the short board.  So the one above it is my point of reference.  Sorry for the confusion. I also appreciate your responses. This is making me think.  Dont worry, I wont turn it on unless I know it's right. 

[tubeswell]

I see.  I'm not using the short board.  ...

Even if you're not using the short board, each output tube really has to have it's own screen resistor (to prevent screen grid failure under heavy signal conditions). So you might as well mount them like that (seeing as how you've got 4 output tubes'n'all) - unless your'e going to somehow mount all 4 screen grid resistors on the main board (which is inferior to mounting them at the tube socket pins - from an RF interference point of view)

[2deaf]

Even if you're not using the short board, each output tube really has to have it's own screen resistor (to prevent screen grid failure under heavy signal conditions). So you might as well mount them like that (seeing as how you've got 4 output tubes'n'all) - unless your'e going to somehow mount all 4 screen grid resistors on the main board (which is inferior to mounting them at the tube socket pins - from an RF interference point of view)

After I made my mistake and removed it, I read the first post carefully and looked at the Hoffman build stuff.  The board for 4 output tubes has four screen resistors on it, so all the OP needed was the info on connecting the grids.

[sjwood3]

Yes.  RF superiority aside (which I'd prefer), that is my question.  I'm trying to keep this build as simple as possible according to Hoffman's specs. Mods can be made later.  I love these responses.  Really have my gears turning!

[Bantam]

Every twin I've ever come across is 4 power tubes already... Is this like an old one with two that's being built up to 4? I've obviously missed something.

[sjwood3]

It's a quad reverb chassis, 4 Power tubes already there. 

[sluckey]

Every twin I've ever come across is 4 power tubes already... Is this like an old one with two that's being built up to 4? I've obviously missed something.

Take a look at Hoffman's AB763 documents. Should cover everything you're missing.

[tubeswell]

Yeah- different AB763 layout options in Doug’s pictures. The layout diagrams show two tubes with choices for grid resistor layouts on sockets or boards, and the notes also discuss 4 tubes.


So this is the Quad Reverb rebuild from that other thread? Now I see where you’re coming from.

[sjwood3]

Yes this is the rebuild that I was taking suggestions on from the other thread.   Sorry I started a new thread!  I'll keep it all to this post.   I can clearly see the layout difference between the short and long boards now.  I'm following the layouts and schematics, but fully understanding how and why it's all happening is crucial for me as well. That's part of the fun. 

With regards to the pin 6 connections for each pair you (sluckey) mentioned above, why is this not covered in the image explanation for the difference between 2 tube and 4 tube (Hoffman Ab763 2 Channel) amps?  Screen shot of the section is what I'm talking about. 

[sluckey]

With regards to the pin 6 connections for each pair you (sluckey) mentioned above, why is this not covered in the image explanation for the difference between 2 tube and 4 tube (Hoffman Ab763 2 Channel) amps?

  Only the Doug knows why???

But I can tell you what will happen if you don't make that connection.   

[sjwood3]

I bet!  I really appreciate everyone's insight.  It's been super helpful.  My board arrives tomorrow.   I'll have more questions I'm sure. 

[sjwood3]

With regards to the pin 6 connections for each pair you (sluckey) mentioned above, why is this not covered in the image explanation for the difference between 2 tube and 4 tube (Hoffman Ab763 2 Channel) amps?

  Only the Doug knows why???

But I can tell you what will happen if you don't make that connection.   

Just want to make sure I followed what you said about jumpers on the power tubes; 3 to 3 on both pairs and 6 to 6 on both pairs.  Haven't hooked up the rest yet, those suckers were hard to make look good!   I installed the tubes while wiring those up so the pins would fit after I'm done. 

[sluckey]

That looks good. This should answer any other questions about output tube wiring that may come up...

[sluckey]

My board arrives tomorrow.   I'll have more questions I'm sure.

You will have to drill mounting holes in your board and the chassis. Do this BEFORE you do anything else to the board. Put the board inside the chassis and move it around slightly until you are satisfied with the position. Figure out where the mounting holes should be so as to not interfere with any existing screws (ie, transformer screws, cap pan screws, etc.) or any other hardware. I suggest 6 mounting holes and standoffs for a board this long.

[sjwood3]

Thanks!    That's a exacty how I envisioned the image I was looking for on Hoffmans site.   I've been following the order of operations he does with his builds.  So I am definitely waiting until my board arrives to move into all pin wiring that goes to the board.  Working on the basic infrastructure for now.  Input jacks are next.   Where in that chassis would you suggest to put a rotary switch for the addition taps on my OT transformer?  Its a 100W Hammond with two additional taps.  4/8/16. The switch is big and beefy, so either there isn't room for it or the taps wont reach it where there is a space for it in the chassis.   Last resort is not place the switch initially and create an exterior box for it later. 

[sluckey]

That impedance selector switch should be mounted very near the speaker jacks. Do not mount it on the preamp end of the chassis!

An alternative would be to use three speaker jacks, one for 4, another for 8, and the third for 16.

If you have the original back plate on the chassis, nothing you do will look very good.

[sjwood3]

With regards to the pin 6 connections for each pair you (sluckey) mentioned above, why is this not covered in the image explanation for the difference between 2 tube and 4 tube (Hoffman Ab763 2 Channel) amps?

  Only the Doug knows why???

But I can tell you what will happen if you don't make that connection.   

I found this link on Doug's site.  https://el34world.com/charts/CommonHookups.htm

At the very bottom there ins an older looking diagram of the same 4 power tube set up.    

[sjwood3]

That impedance selector switch should be mounted very near the speaker jacks. Do not mount it on the preamp end of the chassis!

An alternative would be to use three speaker jacks, one for 4, another for 8, and the third for 16.

If you have the original back plate on the chassis, nothing you do will look very good.

Suppose I used a DP3T switch for impendance like in an Ampeg?  Would fit better somewhere.     is that the same function, but a different shape?

[sluckey]

I would have to see the switch and/or a datasheet to answer that question.

[sjwood3]

I would have to see the switch and/or a datasheet to answer that question.

I'll leave that one for another day.  Likely I'm going to stick with the one output for now.   Once I find the best solution I'll tackle it as an additional modification. 

Next question.   The filter caps.  My initial thought is to wire them in Series with 220K 3 watt Metal films.  I see that depending on the series or parallel arrangements in the "common hookups" section, there is a difference in Voltage.  I'm planning on using the doghouse to house the filter caps.    I can't figure how the B+ makes it from the power transformer to the standby switch by looking at the Hoffman layout.    I get the concept of how it should work.  Could be something simple, but I need some education on that.   The bias tap from the transformer  goes to the bias tap (right?), but where do the two red wires from the secondary go?  On a fender schematic, they go to the diodes  then out to the standby switch, but the bias circuit is on the board on the Hoffman, and I don't see an obvious place for the diodes.   Its a smidge confusing.  Maybe there is one, but I cant find a matching schematic to follow the Hoffman.  What am I missing?   Shouldn't there be 3 diodes off each red secondary line somewhere?   Links or answers welcome. 

[sluckey]

Looks like you need to ask Dougabout the diodes. He may have a small board for them. He may tell you to mount them on a small terminal strip. If you have the original diode board you could use that. Or you could make a simple turret board just for the diodes.
Just send him a pm.

[sjwood3]

Looks like you need to ask Dougabout the diodes. He may have a small board for them. He may tell you to mount them on a small terminal strip. If you have the original diode board you could use that. Or you could make a simple turret board just for the diodes.
Just send him a pm.

Sent him a message.   I have a feeling you're right.  Looking over the schematic, the diodes should be there as the obvious point of rectification.  I have them as well as included in the BOM.   

WHat about this switch in the AC plug hold next to the fuse housing?  Could somehow shield the wires running to it?   Would running those connections by power tubes cause issues?

https://www.tubesandmore.com/products/switch-rotary-impedance-selector-replacement-marshall

[sjwood3]

Looks like you need to ask Dougabout the diodes. He may have a small board for them. He may tell you to mount them on a small terminal strip. If you have the original diode board you could use that. Or you could make a simple turret board just for the diodes.
Just send him a pm.

Sent him a message.   I have a feeling you're right.  Looking over the schematic, the diodes should be there as the obvious point of rectification.  I have them as well as included in the BOM.   

WHat about this switch in the AC plug hold next to the fuse housing?  Could somehow shield the wires running to it?   Would running those connections by power tubes cause issues?

https://www.tubesandmore.com/products/switch-rotary-impedance-selector-replacement-marshall

I figured out the diode situation.  Ordered a board, I'll post pics once I get it wired up.   

[sluckey]

WHat about this switch in the AC plug hold next to the fuse housing?  Could somehow shield the wires running to it?   Would running those connections by power tubes cause issues?

https://www.tubesandmore.com/products/switch-rotary-impedance-selector-replacement-marshall

Looks like a good solution.

Did Hoffman already have a diode board?

[sjwood3]

Yes, he has a bias board that includes a diode section for this type of set up.  I'll obviously leave the bias section alone.   I didn't think to look for it on the bias board.   

How would the Negative feedback need to be connected to that type of ohm selector switch?

[sluckey]

The NFB should be connected directly to the 4Ω lead.

[sjwood3]

That looks good. This should answer any other questions about output tube wiring that may come up...

Here in the two power tube pic. There is a 1k/3W resistor.  The pic you posted was 10k/3W?  Is that a change I need to make or was that a typo? 

[sluckey]

1K/3W is the correct value for a twin reverb.

[sjwood3]

Here's progress.  Getting fun!  I grounded the preamp buss under the the standoff closest to the input Jack.   I'm going for a floating bus instead of a pot soldered bus.     

[sjwood3]

Let's say I changed the value of C22 coming off the phase inverter to .001 from the suggested value of .01.  I ordered one short, but I have an extra .01/630V on hand.   What effects would that have?  Would  it be possibly detrimental to try?

[shooter]

Would  it be possibly detrimental to try

nope,  coupling caps, simple "look", the bigger the cap, the bigger the "bandwidth"

[sjwood3]

My options are .1 or .001 /630V of the yellow Mallorys.   I could order another one...But I already have these parts.  Would .1 be to much?

[shooter]

Would .1 be to much

.1 is usually found more in HiFi, lets the bass through
.001 will tilt treble, so if you use 1 ea   

[sjwood3]

Just found  this schematic.   I guess the .001 will do just fine since it's already there. Hoffman suggests .01 in his layout.