Fender Deluxe Reverb Reissue Hoffman Board Conversion - The Basics

[davegardner0]

First post on here, hopefully this is in the right forum!

I have a Fender Deluxe Reverb 65 Reissue which I picked up for a steal a few months back... The cab/speaker/chassis/transformers are all in good shape but the PCB has some issues. The board has connection issues which I spent a month or two chasing, but the traces are starting to peel and as soon as I fix one issue I create a few more. Instead of working on this PCB more or getting a new one, I thought I might convert it to old-style PTP instead. The hoffman board/parts kit is pretty economical and looks like a fun project! I'm an engineer by day and have done some repair work on solid-state amps in the past, so I think this project is within my abilities although it'll be a learning process.

I've read through all of the documentation on the Hoffman website. Before I order parts and begin I thought I'd ask for some general pointers and things to watch out for from those who have done this sort of thing before. I also have a few specific questions/clarifications needed.

I'm thinking of reusing the transformers and tube sockets (good idea?), so can I just use the Reissue's PCB schematic and PCB layout (I have a service manual) to map out what each wire coming out of the transformers is? Then I can figure out where that would go on the Hoffman board. Also can I just leave the heater and rectifier tube wiring in place as-is?

The current power supply PCB actually looks to be in good shape, is there any harm in just leaving it as-is for now? I can always convert that to PTP later if I want/need to. Or does the Hoffman design use different cap values than the reissue circuit? I know some of the cap values were changed in the reissue.

I'm looking at the .pdf 2-channel AB763 layout document (http://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf), and I see on the schematic that certain components in blue have different values based on which AB763 amp you're building (Deluxe, Super, Twin, etc.). But, the actual board layout doesn't have the components numbers (ex. R11, C32, etc.). So, how do you know what the "blue" components from the schematic correspond to?

Also what are all the wire connections that aren't shown on page of 2 this .pdf? For instance I know the heater wires aren't on there. What else is left out? On a similar note, what about grounding? The RI has some grounding to the chassis? Is that all covered/shown in the .pdf or do I need to ground more stuff?

Well that's all I can think of for now...I thought I'd get all of my questions answered before I order my parts and start so there are no surprises... Thanks!!

[eleventeen]

Here's some pix of work from forum member mscaggs. His builds and pix are super, most of your questions will probably be answered just by looking at his pix. http://p2pamps.com/rmpr

[Mike_J]

There are a few differences between the 6L6 based amps and the 6V6 based amps. The 6V6 amps use higher valued resistors in the power string and therefore have lower voltages on the plates of the preamp tubes versus the 6L6 based amps. There are also differences in the cathode caps for the PI tube. However, if you follow the schematic for the AB763 Deluxe Reverb you will have all the correct values for your amp project. In my opinion initially, the best use of your time would be to take the Fender AB763 schematic and compare it to the layout. What you will find is there are wires coming out from the back of the amp on the layout that get easily explained by looking at the schematic. They mostly relate to wires from the output transformers and choke and from the power caps. Take a yellow highlighter and trace the circuit from the layout to the schematic and you will see that is what is left over. Doug treats the power caps a little bit differently which you will understand much better after you finish highlighting the layout and look at his pictures. He places the power caps between the pots and circuit board. Fender used to put them on a cap board under a little chassis behind the amp.


If it were me I would eliminate all the PCB. You know you have already had problems with the PCB board so why not eliminate the possibility of that being a problem in the future. Not much additional cost or work to do it now. As far as the cost of a new build is concerned you have the majority of that covered. Biggest costs in a build are cabinet, transformers, chassis and speaker. You have all those items covered already.


I would strongly suggest that you purchase the two-channel board from Doug with turrets attached. You don't save much by making the boards yourself. Until you get to the point where you are planning your own builds and layouts there is no sense in making your own boards in my opinion. By using Doug's layout all you have to do is populate the board the way he has done it on his two-channel build. This is a proven layout that has been installed successfully by a number of people.


I know this seems a little confusing but if you approach it systematically it is not that complicated. Good luck and ask questions any time you need help. I ask for help all the time. Sluckey has pulled me out of so many messes. Always seems to know where my problem is.


Thanks
Mike

[Mike_J]

As far as grounding is concerned you only want three grounds in my opinion. One is connected to the input jack ground. You can see the preamp grounds on Doug's documentation and where he places them. You will need a buss bar behind the pots to connect grounds to. A buss bar is just a thick piece of copper wire that many of the preamp grounds are tied to. The ground on the AC wire coming in from the wall gets its own ground close to the point where the AC wire enters. The third is the ground for the power supply side of the amp. That ground point should be close to the power transformer. The power transformer high voltage secondary center tap (red/yellow wire) and the heater center tap (green/yellow wire) should be grounded to that point. You will also ground your power caps tied to the first filter and screen cap at this point. If you are going to isolate your speaker jacks they would be grounded here. If not, the speaker jacks are a forth grounding point. Sometimes you can get away with this but I usually isolate the speaker jacks and ground them with the power components.


I know this seems confusing but it is not that bad if you just think about isolating the power side from the preamp side. That is what this grounding method helps you do. Again just ask questions if you have any.


Thanks
Mike

[davegardner0]

Lots of good info here, thanks!

Mike_J, really good info on grounding, that makes a LOT more sense to me now! 

I'm definitely planning on buying the premade AB763 board. In fact, my plan is to buy this kit, minus the parts I'm going to reuse:
http://hoffmanamps.com/MyStore/PartsListPublic.php?action=public&p=N&f=Amp_AB763TwoChannelParts_DR
So it sounds like this board/circuit isn't 100% exactly like the Fender schematic? (which is fine!)...but as far as the power supply goes are the filter caps just LOCATED somewhere different from the fender design or is the circuit modified as well (I'll have to take a look at the schematics)? I'll have to think about if I want to put the filter caps inside the chassis or track down a filter cap board to go in the cap pan (what's a good source for one of those boards?).

Here's some pix of work from forum member mscaggs. His builds and pix are super, most of your questions will probably be answered just by looking at his pix. http://p2pamps.com/rmpr

Good photos, very helpful, thanks!!! After looking at these photos the actual wiring/construction definitely seems doable to me. It's just a matter of figuring out where everything goes that I'm unsure of, since I don't know a lot about "the way things are always done", etc...

[eleventeen]

I am certain Hoffman sells a cap board for under the "doghouse" as it is often called.


There is a lot of logic going for the "scatter around the amp" mode of ecap location versus "inside the doghouse".


With stiff wires, you can use PC mount caps which cost $1.50 - $2 versus $12+ for big blue Spragues or $3-4 for other axials. No expl needed getting the filtering close to the stages and having shorter ground and B+ connection(s). It's better than old-style Fender, or at least it ought to be.


Yes, it's just the physical locations that are different, the schematic is the same for the most part.

[Mike_J]

Doug uses a different vibrato circuit versus the normal Fender blackface vibrato with the roach. His vibrato is from the 6G16 Vibroverb amp. In my opinion it is a much better sounding vibrato circuit. Otherwise, Doug's board is like the original AB763 circuit as I recall tracing them before I made mine. (Didn't pay attention to the difference in power string resistors when I built mine. Had the higher plate voltages normally associated with the 6L6 amps. Sounded good but I changed two resistors and got it to Deluxe Reverb territory.) If you follow his board and connect the wires exactly like his pictures show you will end up with an excellent amp.


As you mentioned Doug placed his power caps between the pots and circuit board. I believe it is advantageous to tie the ground side of a power cap to the grounds for every part of the circuit the cap feeds. Preamp tubes usually have a cap and resistor going from the cathode to ground. The cathode ground(s) should be attached to the negative side of the same power cap that has the positive side of the cap tied to the tubes plates. It is usually best from my experience to have the cap as close to the other grounds as possible. I am pretty sure that is why Doug did his that way. Plus you don't need to buy a cap can or need a circuit board for the caps.
 
 Another thing I would consider is getting isolation jacks for the reverb and vibrato jacks or you will have one more source of possible ground hum. Send the grounds to the negative side of the reverb and vibrato cathode caps on the board. If you do that there is a lower likelihood of noise in the reverb and vibrato sections. Two common problems of noise and hum relating to a new build are related to the reverb section. The first can be solved by isolating the jacks and the second is proper placement of the reverb pan. Don't think you have to worry about the later since Fender built the original amp and the former will be prevented by isolating the jacks.


Thanks
Mike
 
 

[davegardner0]

Really good info yet again, thanks guys! I never really thought much about grounding schemes before and therefore didn't understand why different grounding schemes aren't all equivalent. Makes sense though, and very interesting!

By "isolation jacks" do you mean that the sleeve isn't grounded by the physical bolting of the jack into the chassis? If so, where could I get such a jack? Does anyone have a Switchcraft Part #?

And by "the roach" I assume you mean the opto-oscillator??? I like that terminology  I guess what that is is a light bulb and a photodiode shrink-tubed together? So crude. I read the bit about how the Hoffman circuit's tremolo works differently - how does it sound different though?

Now as far as the caps inside the chassis that sounds like the path of least resistance, so I'll take it :). My aim is to follow the Hoffman design exactly as I have no reason not too. I guess the only downside here is I'll have to buy new filter caps, but I guess that was unavoidable since the Fender RI uses different values. No big deal. How exactly are the caps secured inside the chassis?

I might use a push-pull SPST pot somewhere on the amp to engage/disengage the bright cap, no issues there, right?

One very interesting part of this project so far is that I'm getting to see exactly which parts of the amp contribute the most to the cost. Like someone said previously I already have all of the expensive parts, which is really great! Well except the labor for assembly and wiring, if I wasn't doing it myself I'm sure that would be a huge contributor to the cost of a PTP amp!

[eleventeen]

Isolation for jacks is usually achieved (one way) by mounting a normal grounded jack inside a two-piece washer set that holds the diameter of the threads off the hole in the sheet metal mounting surface; this implies a bigger hole is drilled in the sheet metal which you may not want to do. It's always something.


The type of chassis-mount RCA jacks used on old Fenders (for the rev & trem) sit in a 3/8" hole, same as a pot or a 1/4" jack. I am sure someone will correct me, but those are not that easy to get. Newer ones often use a 1/4" hole. I do not know, in your case, whether the old RCAs from the donor amp are PC-mount or are the kind that bolt into a 3/8" hole but are now problematic should you wish to lift their grounds.


This is one of those things you have to resolve for your particular project; and the metal you have and the parts you have and how much you want to trick out your project.


There are probably more than 50 things people on this forum have done that are, yes, mods, but fairly minor ones. If you did all 50 of them, that would be hairy.


I don't know if you pretty much wish to build just an ordinary BF DR clone or spend some research time to make some changes. If you *DO* wish to enter the mod mod world of mods, there are lots of cool and useful things available; I will only suggest you do the research BEFORE building it! Eg; finalize the design before execution.

[sluckey]

This link has good pics that may answer a lot of your questions...

     http://el34world.com/Hoffman/AB763Project.htm

The Fender roach is a neon bulb and light dependant resistor (LDR). The neon flashes causing the LDR to change resistance at the flash rate. The LDR shunts the guitar signal to ground to produce the amplitude variation. It sounds a bit choppy. The 6G16 tremolo circuit produces a low freq. sine wave that actually modulates (varies) the bias on the output tube to produce a very smooth (sinusoidal) amplitude variation. It sounds very smooth.

You don't really need isolated input jacks on the AB763 amp. Cliff jacks are built to be isolated and you must provide a ground wire to the sleeve terminal. Hoffman sells shoulder washers (insulating bushings) for the Switchcraft jacks. See this page...

     http://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=MiscHardware&ORDER_ID=!ORDERID!


Since the bright cap actually connects to the volume pot, using a volume pot with push/pull switch makes good sense.

[eleventeen]

Nice, Sluckey. Those are the exact pix he needs.

[davegardner0]

This is one of those things you have to resolve for your particular project; and the metal you have and the parts you have and how much you want to trick out your project.

There are probably more than 50 things people on this forum have done that are, yes, mods, but fairly minor ones. If you did all 50 of them, that would be hairy.

I don't know if you pretty much wish to build just an ordinary BF DR clone or spend some research time to make some changes. If you *DO* wish to enter the mod mod world of mods, there are lots of cool and useful things available; I will only suggest you do the research BEFORE building it! Eg; finalize the design before execution.

Ok so I should have said from the beginning, I'm looking to do a BF DR clone and not go crazy with mods. Not so much because I'm opposed to anything like that but more because I have no idea what I'd pick! The options out there seem overwhelming! This amp is my first foray into anything Fender-style as I've played Marshall and Traynor amps through closed-back cabs predominantly up to this point. When I had the DRRI semi-working with the PCBs I absolutely loved the tone though! For now I want to achieve the PTP version of that tone, see how I like it, then maybe decide on potential mods to this amp or another in the future. I'm a scientist by day so I guess I'm into my quest for tone being "methodical"...

To be honest, I don't even have an expectation that the amp will sound way better with PTP wiring using the same speaker and transformers. I'm in it for the reliability and repairability (I want to gig a lot with this thing) plus I was horrified at how cheap and flimsy the Fender PCB was.
Here are some of the tones I was getting from the DRRI when it was working with the PCBs. It sounded even better with my Strat and G&L but for some reason I used an SG for the clean tone demo.
! No longer available
! No longer available

That all being said I think I'm ready to order my parts and get started on the project! Once I get the new parts I'll pull out the PCB and see what wiring can stay, then decide on where to start.

Oh and one last thing, would you guys recommend replacing the Fender tube sockets or are they okay quality?

[sluckey]

I'd use the Fender sockets unless they get boogered while removing the wires.

[Mike_J]

Grounding schemes are very important for making a quiet amp. You want the grounds to go in the same order you see them in on the schematic. Especially want to isolate the preamp and power amp sides of the amp which you would do if your grounds go in the order you see them in on the schematic.

When I was talking about isolation jacks it relates to two areas. The first would be the speaker jacks. They are 1/4" jacks. As Sluckey mentioned you can get isolation jacks like the Cliff S-H901 or get the Switchcrafts from Doug and use the isolation washers he sells. Probably will need to use a step drill to make the hole a little bigger to accomodate either. You can try to not isolate the jacks and maybe get away with it. However, if you get hum in the amp on your first build it is pretty frustrating. Isolating the jacks I mentioned will reduce the likelihood of having a hum problem.

The other jacks that I would isolate are the reverb and vibrato jacks. They are rca jacks. Tubes and more sell the S-H267R and S-H267W (red and white). Use the red on the reverb return and the white on the reverb send if your wires are color coded. Connect the red wire to the output portion of the reverb pan. You would need two of each.  They require 3/8" holes. Don't know what size the holes in your amp are. May need to use the step drill here as well. Ask questions if you aren't sure.

Yes the roach is the opto-oscillator. Sluckey's description of the way it sounds is pretty accurate. I have two AB763 builds. The build with the roach in it has been disconnected while the Vibroverb vibrato will be kept if that helps at all.

The E-Caps are not secured at all. Just soldered to the circuit board. You could use some silicon to secure the caps to the chassis but that is not essential.

Sluckey's suggestion to get a push-pull 1MA pot is the way I usually go. Just pull the knob on the pot out to get the bright tone.

Definitely have the majority of the cost of the amp already covered from the amp you purchased. My experience is Fender's transformers on the reissue amps are pretty decent. In other words usually don't need to replace them to get good tone.


Thanks
Mike

[davegardner0]

I think I'll order the rubber isolators for the switchcraft jacks, they're cheap so no big deal. As far as the push-pull pot would the one called "CTS amp pots with SPST pull switch - 1ma" on the pots page work? Or do I need to track down one with a solid shaft somewhere? (Stupid question really but it would be easier to order everything from the same place)

[Mike_J]

As far as the push-pull pot would the one called "CTS amp pots with SPST pull switch - 1ma" on the pots page work?

Yes, that is the one you want.


Thanks
Mike

[bnwitt]

To be honest, I don't even have an expectation that the amp will sound way better with PTP wiring using the same speaker and transformers.

Well, having completed numerous re-issue BFDR conversions for customers, let me be the first to say that a PTP conversion amp (whether it be to an original Fender layout or to a Hoffman AB763 and I've done several of both) will leave behind all kinds of noise and artifacts that the re-issue amplifier has inherently.  I'm not talking hiss or hum but low level parasitic noise I can't describe.  You really don't realize this until you A/B a reissue next to a PTP version.  I've heard this with the Fender Re-issue BFDR and the re-issue Bassmans next to converted units and it is astounding.  If you want just pure guitar tone that puts a smile on your face without amp noise, ditch the PCP board.  I am building an AB763 DR for my studio right now and I've chosen the Hoffman layout to get the sweet sounding bias vary tremolo that Sluckey mentioned above.  I've built several Hoffman Vibroverb versions for customers who love them and IMHO they are fine sounding amps that put the re-issue AB763s to shame.  Read the reviews on the Reverb King amp at the link below which are both Hoffman AB763s with 6L6s I built years ago.  Clean tone no noise is described:

http://www.harmonycentral.com/reviews/brand/corona-ampworks


Barry

[davegardner0]

To be honest, I don't even have an expectation that the amp will sound way better with PTP wiring using the same speaker and transformers.

Well, having completed numerous re-issue BFDR conversions for customers, let me be the first to say that a PTP conversion amp (whether it be to an original Fender layout or to a Hoffman AB763 and I've done several of both) will leave behind all kinds of noise and artifacts that the re-issue amplifier has inherently.  I'm not talking hiss or hum but low level parasitic noise I can't describe.  You really don't realize this until you A/B a reissue next to a PTP version.  I've heard this with the Fender Re-issue BFDR and the re-issue Bassmans next to converted units and it is astounding.  If you want just pure guitar tone that's puts a smile on your face without amp noise, ditch the PCP board.  I am building an AB763 DR for my studio right now and I've chosen the Hoffman layout to get the sweet sounding bias vary tremolo that Sluckey mentioned above.  I've built several Hoffman Vibroverb versions for customers who love them and IMHO they are fine sounding amps that put the re-issue AB763s to shame.  Read the reviews on the Reverb King amp at the link below which are both Hoffman AB763s with 6L6s I built years ago.  Clean tone no noise is described:

http://www.harmonycentral.com/reviews/brand/corona-ampworks


Barry

Aw well now you've got me excited! This is going to be awesome! And now that you say it the pcb amp did have a lot of strange little noises which I thought were just due to an old amp design.

I ordered my parts this morning, looking forward to getting started on this project!

[p2pAmps]

Here's some pix of work from forum member mscaggs. His builds and pix are super, most of your questions will probably be answered just by looking at his pix. http://p2pamps.com/rmpr

New link fellas

http://www.p2pamps.com/drri/

[davegardner0]

So last night I started on the project by identifying the wires on the components that are going to stay (transformers, on/off switch, etc), then I removed the PCBs. I ran into a few things I wasn't sure of so I thought I'd ask.


First of all, while mapping out the taps of the power transformer I noticed a slight difference between what's shown on the DRRI service manual schematic and the Hoffman layout schematic.
For the most part the fender transformer follows the color coding on the Hoffman drawing. I can identify the leads for the primary coil (black & white), the 5 volts (yellow pair), the High voltage (red pair), and the heater wires (green pair). However the other two wires are what's confusing me. The Hoffman diagram shows that there are two taps in the middle of the red wire pair's coil (purple and black wires), but the fender shows 2 wires on their own coil, with one jumped to the middle of the red coil (red/yellow and red/blue wires).




Are these wire pairs equivalent? Does (fender)red/yellow = (hoffman)black and (fender)red/blue = (hoffman)purple ? If not how would I need to modify the Hoffman diagram to utilize the fender PT?

Ok, next question: are the transformer coils polarized? For instance on the PT does it matter which of the red wire pair goes where? What about the choke, is it directional? On the OT does the Blue Pin 3 need to go on the + speaker side?


Next question: I see on the Hoffman .pdf document one of the PT wires goes to a box that says "Bias Tap". Does this mean that this wire connects to the circuit board at the "Bias Tap" box on there just like the A, B, C, D points? If I do banana jack bias points, is this also where one gets connected? Is the 2nd one to chassis ground? OR are each of the "Bias Tap" boxes separate test points?

Related question, on the Hoffman diagram I see two "Board A" boxes and two "A" boxes. Do all four of these get connected together?

Thanks for the help!

References:
Fender DRRI Service Manual: http://ampwares.com/schematics/65_Deluxe_Reverb_RI.pdf
Hoffman Layour .pdf: http://el34world.com/Hoffman/files/Hoffman_AB763_2.pdf

[sluckey]

Are these wire pairs equivalent? Does (fender)red/yellow = (hoffman)black and (fender)red/blue = (hoffman)purple ?

That's correct.

Ok, next question: are the transformer coils polarized? For instance on the PT does it matter which of the red wire pair goes where? What about the choke, is it directional? On the OT does the Blue Pin 3 need to go on the + speaker side?

No, no, no, and no. The OT blue wire connects directly to pin 3 of a 6V6 socket. That should already be made. The OT brown wire should be connected to pin 3 of the other 6V6. The OT blue and brown wires 'may' need to be swapped later on, so don't trim them to finished length initially.

Does this mean that this wire connects to the circuit board at the "Bias Tap" box on there just like the A, B, C, D points?

Correct. If you want to add bias test points they will connect to pin 8 of each 6V6. IOW, you'll have two bias test points. Some people also provide a third black test point for ground. I would use pin jacks, not banana jacks.

on the Hoffman diagram I see two "Board A" boxes and two "A" boxes. Do all four of these get connected together?

Yes

[davegardner0]

Thanks for the info, all really good!!

Ok, next question: are the transformer coils polarized? For instance on the PT does it matter which of the red wire pair goes where? What about the choke, is it directional? On the OT does the Blue Pin 3 need to go on the + speaker side?

No, no, no, and no. The OT blue wire connects directly to pin 3 of a 6V6 socket. That should already be made. The OT brown wire should be connected to pin 3 of the other 6V6. The OT blue and brown wires 'may' need to be swapped later on, so don't trim them to finished length initially.

So how do you know if these will need to be swapped or which way to wire them? I'd imagine this determines which way the speaker moves for a positive or negative voltage?

Does this mean that this wire connects to the circuit board at the "Bias Tap" box on there just like the A, B, C, D points?

Correct. If you want to add bias test points they will connect to pin 8 of each 6V6. IOW, you'll have two bias test points. Some people also provide a third black test point for ground. I would use pin jacks, not banana jacks.

Ok, so those "Bias Tap" boxes have nothing to do with bias testing and they could have just been called box "E"? And if I make some bias test points I'll need to wire them up to pin 8 instead? How many bias test points would there be in total then, 2 or 3? One from each 6V6's pin 8 and then do I also need a ground?

[mresistor]

You leave the OT wires to the plates of the 6V6 long enough to swap because if you get them out of phase the amp will make some really nasty noise and that tells you you need to swap them.


The BOM will describe what value components are used for each different configuration of the AB763  download it and look at it   you'll see


The bias tap box on Dougs layout is where the bias wire from the Power Transformer connects. It's a bias tap on the secondary winding of the transformer.

[sluckey]

So how do you know if these will need to be swapped or which way to wire them?

You won't know for sure until you turn the amp on. We have a 50/50 chance of getting it right. That's why I suggested don't trim to finished length until you positively know they are connected right. This deals with the negative feedback phase. Get it wrong and you will have a positive feedback circuit. An AB763 will usually squeal or howl loudly if the phase is wrong.

One from each 6V6's pin 8 and then do I also need a ground?

You don't actually need the ground test jack but some people find it convenient. You could just use a gator clip to connect the black meter lead to chassis. In fact, you don't really 'need' any test jacks. You can use pin 8 of the tube as the test point. Or clip a lead to one of the on-board 1Ω resistors which connect to pin 8. The test jacks are a convenience only. But if you are nervous or have shaky hands, I suggest you install test jacks.

Ok, so those "Bias Tap" boxes have nothing to do with bias testing and they could have just been called box "E"?

Correct. There is no significance to the boxes other than to quickly draw your attention to what's written inside the box. Hoffman simply uses the box as an annotation callout. The phrase 'bias tap' is more meaningful than 'E', especially when it refers to several different pages. Hoffman uses 'bias tap' to indicate the AC source for the bias supply. This annotation convention works well. Others use different conventions. Here's my style of annotation. Notice I did use 'E' to represent the bias voltage points. Using different colors helps to quickly show where those points are connected on the schematic and eliminates the need to clutter the schematic with long connecting lines.

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf

[davegardner0]

So how do you know if these will need to be swapped or which way to wire them?

You won't know for sure until you turn the amp on. We have a 50/50 chance of getting it right. That's why I suggested don't trim to finished length until you positively know they are connected right. This deals with the negative feedback phase. Get it wrong and you will have a positive feedback circuit. An AB763 will usually squeal or howl loudly if the phase is wrong.

One from each 6V6's pin 8 and then do I also need a ground?

You don't actually need the ground test jack but some people find it convenient. You could just use a gator clip to connect the black meter lead to chassis. In fact, you don't really 'need' any test jacks. You can use pin 8 of the tube as the test point. Or clip a lead to one of the on-board 1Ω resistors which connect to pin 8. The test jacks are a convenience only. But if you are nervous or have shaky hands, I suggest you install test jacks.

Ok, so those "Bias Tap" boxes have nothing to do with bias testing and they could have just been called box "E"?

Correct. There is no significance to the boxes other than to quickly draw your attention to what's written inside the box. Hoffman simply uses the box as an annotation callout. The phrase 'bias tap' is more meaningful than 'E', especially when it refers to several different pages. Hoffman uses 'bias tap' to indicate the AC source for the bias supply. This annotation convention works well. Others use different conventions. Here's my style of annotation. Notice I did use 'E' to represent the bias voltage points. Using different colors helps to quickly show where those points are connected on the schematic and eliminates the need to clutter the schematic with long connecting lines.

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf

Ok NOW I follow you on everything. The positive/negative feedback thing makes total sense. And I was overthinking the "Bias Tap" annotation.
I have a eurotubes octal bias probe that I usually use to bias amps but I was thinking the test points will be easier since the tubes don't need to be removed. I guess to use them I'd read a voltage to ground though, instead of current, right? And the two different points are for reading the voltage on each tube? Is that the "plate voltage"?

[davegardner0]

The OT blue wire connects directly to pin 3 of a 6V6 socket. That should already be made. The OT brown wire should be connected to pin 3 of the other 6V6. The OT blue and brown wires 'may' need to be swapped later on, so don't trim them to finished length initially.

Oh and I ended up ordering new tube sockets...changing from the plastic Fender sockets to ceramic sockets seems like a good idea. Plus some of the intermittent/connection issues the amp was having changed when wiggling the tubes, so some of the sockets might have had some bad connections. When I was unsoldering things I noticed a lot of the pins in the tube sockets are very wiggly and loose - that doesn't seem like a good thing to me.

[sluckey]

I guess to use them I'd read a voltage to ground though, instead of current, right? And the two different points are for reading the voltage on each tube? Is that the "plate voltage"?

That's correct. But notice that the voltage you read will be across a 1Ω resistor. So, the current through that resistor will equal the voltage across the resistor divided by the value of the resistor. That 1Ω resistor is called a current sense resistor because the voltage across it represents the current through it. Dividing by 1 is easy. IOW, if you measure 25mV across the 1Ω resistor, there will be 25mA flowing through the resistor. This is called the cathode current and is considered the same as the plate current. Plate current is actually slightly higher because some of the cathode current flows into the screen grid.

The plate voltage is different. That will be measured from pin 3 of the output tubes with reference to ground and will be approx. +400vdc for your amp.

[sluckey]

When I was unsoldering things I noticed a lot of the pins in the tube sockets are very wiggly and loose - that doesn't seem like a good thing to me.

The pins on the new sockets will be wiggly too... until you plug the tubes in.   

[davegardner0]

When I was unsoldering things I noticed a lot of the pins in the tube sockets are very wiggly and loose - that doesn't seem like a good thing to me.

The pins on the new sockets will be wiggly too... until you plug the tubes in.   

Right, "solder with tubes installed" is standard procedure?

[Mike_J]

Drew the attached layout showing how the bias tip jacks are installed. As Sluckey mentioned you don't have to have these jacks. They just make it easier to bias power tubes. The make biasing power tubes hands free except for turning the bias (trimmer)pot.


If you look at the Hoffman main circuit board you will see two one ohm resistors on it. You want the red tip jacks installed somewhere between pin 8 and the one ohm resistor on the circuit board. You need one jack for each tube. The black jack is tied straight to ground.


The bias tap point on the power layout and the main circuit board would be wired together as previously mentioned. The 10KL trimmer shown is to make it easier to bias your tubes. The box marked "Bias Range" is there to let you know if you can't get the range you need to bias your tubes you change that resistor to change the range. It is not a today problem for you. Once you build your amp we can help you with start up procedures that would cover the pin 3 phasing mentioned earlier, biasing your power tubes, going through the schematic in comparison with your amp with a yellow highlighter and a few other things. These are not a problem for today though. All you need to do now is build the circuit board like the layout documents and pictures show and install the wires to the tube sockets exactly the way the layout and pictures indicate. If you do that you will have a successful build.


Thanks
Mike

[davegardner0]

It is not a today problem for you. Once you build your amp we can help you with start up procedures that would cover the pin 3 phasing mentioned earlier, biasing your power tubes, going through the schematic in comparison with your amp with a yellow highlighter and a few other things. These are not a problem for today though. All you need to do now is build the circuit board like the layout documents and pictures show and install the wires to the tube sockets exactly the way the layout and pictures indicate. If you do that you will have a successful build.

My thoughts exactly! Right now I'm at step "figure out where all the wires need to go", next is step "put the wires where they should go". 

[Mike_J]

One thing I learned the hard way. Best to use a sacrificial tube in the socket you are soldering to. Will need an old preamp and an old power tube. When you are soldering pins on a socket you can flow solder into the socket pin if no tube pins are present. Not much more frustrating than thinking your amp is all ready to button up but you can't get a tube in one of the sockets because one of the holes is full of solder. Especially true when you are new to this and don't have any spare sockets. Have to wait for a new socket to arrive before you can fire the amp up for the first time. Not a pleasing moment.


Thanks
Mike

[sluckey]

Right, "solder with tubes installed" is standard procedure?

Not for me!

When you are soldering pins on a socket you can flow solder into the socket pin if no tube pins are present.

If this happens to you I suggest you 'may' be using too much solder and/or using cheap sockets.

[Mike_J]

Right, "solder with tubes installed" is standard procedure?

Not for me!

When you are soldering pins on a socket you can flow solder into the socket pin if no tube pins are present.

If this happens to you I suggest you 'may' be using too much solder and/or using cheap sockets.

I doubt very much that I need the tubes in the sockets today. Much better at flowing solder than I was years ago. The old tubes in the tube sockets are an ounce of prevention for me. Don't know if I haven't had any problem since using the old tubes in the sockets is due to learning to solder better, not using cheap sockets or the old tubes themselves. Only know that I haven't had a socket problem, at least related to solder, since I started using the old tubes. Sure everyone has their own system. Mine stems from a problem I encountered in one of my first builds. Maybe my first build. It was quite a while ago. Haven't had the problem since using the old tubes. Just passing along my experience. 


Thanks
Mike

[davegardner0]

Well I do have a scrap 12AX7 and a bad EL34 laying around, so I'll pop those in to solder. Sounds like there's no reason not to...

[sluckey]

Solder will stick to octal tube pins. That's how the wires coming through the glass are attached to the socket pins. If you shove enough solder down into an octal socket, you may end up with the tube soldered to the socket. 

[Mike_J]

Solder will stick to octal tube pins. That's how the wires coming through the glass are attached to the socket pins. If you shove enough solder down into an octal socket, you may end up with the tube soldered to the socket. 

Haven't ever had that happen but Sluckey probably has a point. Built my first amp about 15 years ago after having built a car and house from scratch. Said I would never ever build another car or house but do like amp building, my wife would say maybe too much. The obvious key is proper soldering technique. When I first started there were no forums to get help from. Had never done any electrical soldering in my life. So a lot of what I do is because I messed something up in the past and tried to learn from the mistake. Still make many mistakes, thankfully most of which Sluckey got me out of.


To make a long story short, if you can consistently flow just enough solder on the pins to make a proper connection between the wire and pin then there is no need for an old tube. After 15 years of soldering I doubt that I need them. Just remembering when I was at your experience level and what I think helped me.


Thanks
Mike

[Willabe]

Right, "solder with tubes installed" is standard procedure?

Yes, but with an old/bad/dead tube.

[davegardner0]

Got my parts today!

-Finished removing the old components
-Drilled the Chassis and Turret Board for Mounting
-Drilled out the Pot Holes that Needed it (For the CTS Pots)
-Mounted Pots, Tube Sockets, RCA Jacks
-Started Infrastructure Wiring (Reverb Transformer, OT, Rectifier Tube)

A question I came across as I was working:
-Is there any standard wire color convention that I should be using? Also what about which wire gauges to use for which purpose inside the amp? I received 20 gauge solid core in Red, White, Black, and Green, plus 18 Gauge Stranded Core also in the same colors, plus some back sheided wire (I know where that one is used)

[davegardner0]

A few photos:

All torn down




As it sits now:



There were some comments on amount of solder on the tube socket tabs. How do these connections look?

[Mike_J]

I have always heard that it is best to twist the AC secondary wires tightly together. That would be the yellow, red and green wires coming out of your power transformer. Looks like you may be a little gun shy about the amount of solder you use on the socket pins. Impressed with how little you used. Probably my fault for being so neurotic about getting solder in the pins. I am sure there are others on the forum that are more classically trained in soldering technique than I am but I try to flow enough solder to fill the hole in the pin.


Thanks
Mike

[Mike_J]

According to PRR you want to keep all "dirty" wires away from other wires. Dirty wires would include AC wires and the white DC wire you have coming off pin 8 of the rectifier tube socket. The white wire is dirty because it has not been filtered yet (a wire gets filtered by an electrolytic cap). So you want to keep the AC wires and the white wire away from any wires that have been filtered. If the white wire has to cross another wire you want it to cross at a 90 degree angle.


The AC main line coming in the amp should also be isolated as much as possible from the other components (power tubes). I would do this by grounding the green ground wire close to where it comes through the chassis. Run the black and white wires in the corner of the chassis. You will have to carefully strip back the sheathing.


Thanks
Mike

[HotBluePlates]

... Well, having completed numerous re-issue BFDR conversions for customers ...

Oh, hell!! I missed an appearance of an Amp-Mod Emeritus!! Good to see you Barry!

[HotBluePlates]

... All torn down ...

Might I suggest... Now that you have the original board out, locate the new board's position in the chassis & drill the holes. It will be easiest now before you get any wiring solidified and subsequently in your way.

[Mike_J]

As far as wire is concerned I try to use black wire for grounds. Would use the 20 gauge solid for the main board. Could use red for all the plate wires (preamp pins 1 and 6).  Would use the white wire for the cathodes (preamp pins 3 and 8). Use the green or black shielded wire for the grids (preamp pins 2 and 7).


I like to use 18 gauge stranded for wires carrying HT voltage (use red) and ground wires (again use black). I use 18 gauge stranded for the power tube heater wires (power tube pins 2 and 7). Have heard it is important to keep the wires the same for pins 2 of both power tubes and pins 7 of both power tubes so I use green and white wires for the power tube heater wires to make it easier to keep track of which is which. After you strip the heater wires, or any stranded wire you want to put through a hole, you will want to tin them (put a little bit of solder on the end of the wire so you don't get any straglers. Straglers can cause misery).


You can use the solid core wire for the preamp tube heaters (Pin 9 and combine pins 4 and 5). I try not to use solid core for heaters because I had one break when I moved it once which caused some pain. Sluckey helped me figure that one out. This is probably more neurosis on my part though. It is important that heater wires are wound tightly together so as to reduce the possibility of hum (this is why I told you to twist your AC wires). Twisting AC wires tightly can significantly reduce hum.


Thanks
Mike

[Mike_J]

Some wiring tips I have learned over the years. Supposedly it is best to keep grid wires as short as possible. The grid wires carry very small voltages. You want to keep them as far away from the plate (that carry high voltages) or cathode wires as possible. If they cross it should be at a 90 degree angle. There is some square rule that I will probably butcher but it said something like distance squares the reduction in cross-talk between wires. So wires that are twice as far apart have 1/4 of the cross-talk. Three time as far apart have 1/9th the cross-talk and so on. Don't forget that you have three dimensions. Pictures and layout diagrams usually only show two dimensions. Use all three dimensions to your advantage.


If you want to wire a turret according to NASA standards a wire should go around a turret between 180 and 270 degrees. There should not be any more exposed wire (unsheathed after you strip it) any farther from the turret than the diameter of the wire. If you are wiring a number of turrets together you can wrap the wire all the way around the turret but you should leave a little slack (they call it stress relief) in the wire. Probably don't plan on having your amp blast off into space but if you follow those rules I think it probably would meet NASA standards for going into space. (May be overkill). Doug has a good video on wiring turrets somewhere in his library.


Heaters for an AB763 Fender amp should be run above the tube sockets. When they go into the sockets they should go directly down into the center of the tube sockets (to reduce interference with the other wires connected to the socket) and then to either pin 9 or pins 4 and 5 combined. I usually use a cutoff piece of resistor or capacitor lead to connect pins 4 and 5 together before I wire the sockets. Works best for me. Make sure that cutoff piece does not touch the chassis or you could fry your PTs heater supply. Would make for a bad day.

Thanks
Mike

[davegardner0]

Big clarification needed, I only wired up the rectifier side of the PT, which is why the ac in wires look so messy. Good thinking about moving the ac in's ground closer to the side of the chassis though. Currently it's ground is between the board and the pt, I think I'll use that one for the board's ground and add a new ground lug near where the ac wire comes in. I do have the PT's red and yellow wire pairs twisted, but I need to be more careful with the route of the wire from the rectifier to the standby switch.

Also I did mount the turret board and finish the rest of the drilling before I installed everything. Just took it back out after I got all of the standoffs positioned because I'm not ready to wire stuff to it yet. And I'm glad I did...much easier to clean out all of the metal shavings from the chassis with nothing in it.

[davegardner0]

I have always heard that it is best to twist the AC secondary wires tightly together. That would be the yellow, red and green wires coming out of your power transformer. Looks like you may be a little gun shy about the amount of solder you use on the socket pins. Impressed with how little you used. Probably my fault for being so neurotic about getting solder in the pins. I am sure there are others on the forum that are more classically trained in soldering technique than I am but I try to flow enough solder to fill the hole in the pin.


Thanks
Mike

Well its my first tube amp build but I'm not new to soldering 
I've wired up a ton of guitars plus random other electronics and repairs.

[davegardner0]

Some wiring tips I have learned over the years. Supposedly it is best to keep grid wires as short as possible. The grid wires carry very small voltages. You want to keep them as far away from the plate (that carry high voltages) or cathode wires as possible. If they cross it should be at a 90 degree angle. There is some square rule that I will probably butcher but it said something like distance squares the reduction in cross-talk between wires. So wires that are twice as far apart have 1/4 of the cross-talk. Three time as far apart have 1/9th the cross-talk and so on. Don't forget that you have three dimensions. Pictures and layout diagrams usually only show two dimensions. Use all three dimensions to your advantage.

Thinking back to Physics in college: The cross-talk is due to induction from the changing magnetic fields created by the high voltage wire. The magnetic field will decay with 1/r^2 where r is the distance from the wire (exponential decay). So your numbers are correct as far as wire distances. Additionally maximum induction occurs when the wires are parallel and virtually no induction occurs when the wires are parallel (crossed). The idea of a twisted pair is that the wires have opposite currents thus the magnetic fields they create are opposite and cancel out.

[davegardner0]

I finished up the power transformer wall-side wiring, and also reorganized those wires a bit based on the tips here. I moved the AC cord ground to one of the PT attachment screws (and now I can re-use the screw where it used to be grounded as my board ground). I swapped out the B+ wire for 18-gauge stranded wire with red insulation, and put it in the corner of the chassis.