1976 Boogie SP3 restore help

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I wouldn’t use carbon composite for screen resistors, I personally prefer metal oxide as they are very durable.

Randall Aiken on resistors

I had carbon composite resistors in my Mark 1 amp and the resistor values drifted up as high as 2K, it’s amazing the CC resistors in your amp were so stable. Don’t order 0.047uF polystyrene caps (715/716) get polyester caps 225 or 6PS etc.

I’d be inclined to get rid of the four Dale resistors and just use 1 watt carbon film resistors. I have yet to see a You Tube clip where they demonstrate the tonal difference between resistor types. Carbon Composite resistors are notoriously noisy, you don’t want them in a high gain or early stages of an amp as the noise is amplified throughout the amp.

Coupling the signal from the gain stage to the normal input of amp is done with a 0.01uF cap. You can use anything from 0.001uF to 0.01uF in the amp. It depends on the bass response you are after.

Regards

Mark

Mark,
Why do you use polyester over polystyrene? Is it for tone? I think of polyester as the cheaper both in price and quality.

Those CC screens are "new" Allen Bradley's. I get that the carbon comps are inferior in every way but one. Dissipating energy surges however is an advantage that gets overlooked in the noise/quality discussion. I am about to change the voltage dropping film's to CC on the PS board.

The metal films (could have been oxide but I think film) I pulled tested great. I am not smart enough to say the energy surge advantage is worth having but all my Marks except this one use CC screens.

Since my signal coupling cap is right at the upper range of .001-.01uF, does that mean I'm at the tightest bass response?

Carbon or metal film wouldn't look as out of place as the Dale plate R's. Not that I mind Dale but the 50k vs 82k ohm still has me scratching my head.

 

[electric mayhem]

For what it's worth, the screen resistors Mike B used for my IIB were the dark grey Vishay wirewounds. 470 ohm 3.75w

As for the reverb being factory, I still think it's unlikely. Certainly possible that it was modified/repaired more than once though.

Also, there's an SP-3 (August 1977) on amparchives. Can't tell much from the picture, but you can see what kind of components were being used. Looks like this one mostly has carbon comps

View attachment 820

My IIB K12x with SP-8B has 5W cement wirewound. They don't dissipate energy as well as CC even though they are considered accurate. Upping the power rating to 3.75-5W likely makes that a moot point. I've wondered about added inductance and should look it up.

I'd like to know if the smaller brown Mallory signal coupling cap is .01uF/400V in that pic.

I think I'm missing a .047uF to the right of V2- I see a cut leg there. And I thought I was missing the large filter can on the preamp board but I see this pic has none. The Mark I RI schematic shows 60V/350V which I see in bluebug's '78 pics. Is the preamp filter optional???

Btw, did Mike change your power supply board? Two 40uF instead of the last voltage drop 30uF?

 

[electric mayhem]

I've had a preamp CC crumble in a green stripe by the bridge rectifier and either screens and/or heater CC's break with touches. I know they have downsides and could be especially suspect in high humidity areas.

I bought a emerald road king 2x12 cab that came from the Lone Star coast (with a sidewinder I still haven't listened to) that may as well have been stored in the gulf. Every single piece of metal down to the corner tacks had to be removed. If I lived in that environment I would avoid CC's altogether.

I think with those V1 Dale R's it's time to dig up the fetron/7025 differences and see where the amp stands currently. The 7025 marking on the chassis points to the preamp modified for no fetron but this is a learning process for me.

Is there a PS-3 schematic out there? The Mark I RI one helps but if there isn't one I'll have to draw one so I can better visualize the components, traces and flow.

The Mark I RI schematic states "4 stage" gain. Is that the case with my amp? And to jump ahead in class how many cathode followers are in this circuit? Does the PI implement a differential amp for noise?

 

[James]

My IIB K12x with SP-8B has 5W cement wirewound. They don't dissipate energy as well as CC even though they are considered accurate. Upping the power rating to 3.75-5W likely makes that a moot point. I've wondered about added inductance and should look it up.

I'd like to know if the smaller brown Mallory signal coupling cap is .01uF/400V in that pic.

I think I'm missing a .047uF to the right of V2- I see a cut leg there. And I thought I was missing the large filter can on the preamp board but I see this pic has none. The Mark I RI schematic shows 60V/350V which I see in bluebug's '78 pics. Is the preamp filter optional???

Btw, did Mike change your power supply board? Two 40uF instead of the last voltage drop 30uF?

I'm not sure. The 76 I mentioned earlier only has the five big electrolytics on the separate board. So it might not be missing.

On the IIB? It was already redone with F&Ts from before (I sent it to Mike for the FX loop mod) but they're the stock values.

 

[electric mayhem]

I'm not sure. The 76 I mentioned earlier only has the five big electrolytics on the separate board. So it might not be missing.

View attachment 821

On the IIB? It was already redone with F&Ts from before (I sent it to Mike for the FX loop mod) but they're the stock values.

Good to know the big can filter may not be stock to the 1st Mark. But then the reissue put it in there so does it help or I am I trying to compare apples to oranges circuit-wise?

Do you have a pot on your PS board? I see the smaller e-caps have been replaced on your PS board but whether they are 47uF or 50uF now, do you know why some Marks have 1 of these and some have 2? Originally they were the long skinny light blue Mallory 50uF/75V.

I ordered e-caps off ebay someone had made at Supertech in Taiwan. I'm not familiar but I'll test them and compare to the 47uF IC ones I have from Mesa. This Mallory tested 57.5uF anyway, not bad at 15% for an amp that's seen 5 decades but I want to have some on hand when I test them on the old Marks around here.

 

[James]

Good to know the big can filter may not be stock to the 1st Mark. But then the reissue put it in there so does it help or I am I trying to compare apples to oranges circuit-wise?

Do you have a pot on your PS board? I see the smaller e-caps have been replaced on your PS board but whether they are 47uF or 50uF now, do you know why some Marks have 1 of these and some have 2? Originally they were the long skinny light blue Mallory 50uF/75V.

I ordered e-caps off ebay someone had made at Supertech in Taiwan. I'm not familiar but I'll test them and compare to the 47uF IC ones I have from Mesa. This Mallory tested 57.5uF anyway, not bad at 15% for an amp that's seen 5 decades but I want to have some on hand when I test them on the old Marks around here.

I imagine it depends if there are other differences that make it necessary. I'd be interested to see just how much variation there is in a few different SP-3 amps.

Yes, that one has a bias pot. I'm pretty sure the smaller ones are 47uf, but were 50uf originally. It would probably vary depending on the parts they had available. It seems like the single cap was more common on earlier amps though.

 

[mark1406]

Mark,
Why do you use polyester over polystyrene? Is it for tone? I think of polyester as the cheaper both in price and quality.

Those CC screens are "new" Allen Bradley's. I get that the carbon comps are inferior in every way but one. Dissipating energy surges however is an advantage that gets overlooked in the noise/quality discussion. I am about to change the voltage dropping film's to CC on the PS board.

The metal films (could have been oxide but I think film) I pulled tested great. I am not smart enough to say the energy surge advantage is worth having but all my Marks except this one use CC screens.

Since my signal coupling cap is right at the upper range of .001-.01uF, does that mean I'm at the tightest bass response?

Carbon or metal film wouldn't look as out of place as the Dale plate R's. Not that I mind Dale but the 50k vs 82k ohm still has me scratching my head.

I suggested the 418 as they were in my Mark 1. The Dumble cloners use 6PS as a substitute for 418’s etc. The 715/716 poly have a reputation for being brighter than polyester caps. I haven’t A/B the caps in an amp, but I thought I heard a difference in the output stage 0.1uF caps.

Your preamp is running in class A so surges aren’t likely. The 50K and 82K would be applicable to a Fetron and not a 12AX7.

Unless there is a large difference in the value of the cap, you aren’t likely to hear the difference.

Regards

Mark

 

[electric mayhem]

Thanks Mark. Brighter isn't typically what I go for. I better go further down that rabbit hole. As for that signal coupler I don't like sloppy but the tightest bass response isn't my goal either. I'll leave the Mullard in since it's spec on and for future consideration maybe split the .001-.01uF range.

My gut reaction for metal film was you can do things like change the values of components that affect the f-response or spend some money on pre-tubes that can add or detract from tone in significant ways. I doubted the tonal significance, at least to my ears, of the two poly's and so going with the best electronic, not signal reputation was the way to. So I was thinking anyway.

I've been wanted to try an old Mullard 12ax7 even if I have to rob one of my mics for V1 or V2 where I would expect to hear a difference. But whether I can hear it or not if polyester was the standard it would be stupid to get romantic about CC's and gloss over polyester.

The .1uF radial films I used were the 630V ones from the mesa store. I assumed the met poly stood for polystyrene but idr looking it up so idk which poly they are off hand. With the condition I got this amp I figured I would go further in making it more of a "new amp" than typical servicing so I swapped every .1uF after finding a shorted one. Maybe I'll order both poly in .047uF and just have them on hand.

I see Mouser doesn't have the 3 smaller axial e-caps needed for EQ inductors on my IIB. I'll check digi-key and a couple others but if anyone has a source they like lmk. Looks to me like 3/50, .47/35, .22/35, .15/35 and .033/35.

 

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I too would like to see SP-3 variations since they seem to be there.

I'm surprised given the efforts to enhance this beast V1 is still optimized for fetron.

I put in CC's on the power board. The RI schematic spec was 10k ohm on the 2W final V-dropper and testing confirmed it. I tested a few 1W and 2W for the second V-drop. It was a 5.8k film there and I'm thinking, maybe wrong, that it is supposed to be 5.6k. The old tested 5.06K ohm. The 1st couple 1W CC's tested over 6K, not overly bad but the 2W ones were very close so I put a 2W that tested 5.78K ohm in.

The R across the 50uF/75V was actually 3 R's in parallel. It measured in at 3.23K which was not what I expected. The RI schem. reads "factory select".

 

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I looked up the .1uF/630V from the mesa store which is not to say they don't source what's available at the time.

But given the recent IC embrace from petaluma it's no surprise to see it's a IC 104k which looks like polyester. Good call Mark.

 

[mark1406]

I too would like to see SP-3 variations since they seem to be there.

I'm surprised given the efforts to enhance this beast V1 is still optimized for fetron.

I put in CC's on the power board. The RI schematic spec was 10k ohm on the 2W final V-dropper and testing confirmed it. I tested a few 1W and 2W for the second V-drop. It was a 5.8k film there and I'm thinking, maybe wrong, that it supposed to be 5.6k. The old tested 5.06K ohm. The 1st couple 1W CC's tested over 6K, not overly bad but the 2W ones were very close so I put a 2W that tested 5.78K ohm in.

The R across the 50uF/75V was actually 3 R's in parallel. It measured in at 3.23K which was not what I expected. The RI schem. reads "factory select".

I ditched the factory select resistors and put in a ten turn pot to set the bias. This worked for me.

Regards

Mark

 

[electric mayhem]

Are we biasing the 1st B+?

I think the RI schematic calls for 440V before 5.6k ohm voltage drop. Is that what we are dialing in and is that the correct voltage for the original Boogie? Do you know what value pot you have there? 3k-6k range?

The IIB coli's I have open are double 50uF/75V and 1W 5.6K ohm.

So this is a 4x 6L6 class A, eh? It should compliment my Maverick and blue angel nicely and be a monster. Maybe I'll add 4x EL84's and a switch like the BA and call it synchroclass. Patent applied for.

Does that mean the Heartbreaker and SOB are class A also? If so I have a 6x 6L6 class A.

 

[mark1406]

Are we biasing the 1st B+?

I think the RI schematic calls for 440V before 5.6k ohm voltage drop. Is that what we are dialing in and is that the correct voltage for the original Boogie? Do you know what value pot you have there? 3k-6k range?

The IIB coli's I have open are double 50uF/75V and 1W 5.6K ohm.

So this is a 4x 6L6 class A, eh? It should compliment my Maverick and blue angel nicely and be a monster. Maybe I'll add 4x EL84's and a switch like the BA and call it synchroclass. Patent applied for.

Does that mean the Heartbreaker and SOB are class A also? If so I have a 6x 6L6 class A.

The early amps are all class AB. The bias control puts a negative voltage on the grids of the output valves.

I can open the amp later and show you where I put the bias pot.

Regards

Mark

 

[James]

That's interesting. I know there was a 3k resistor across the bias caps in a 78 I sold a couple years back, but I'm not sure if it was stock - it also had a poorly installed bias pot.

I don't know about the Coliseum, but I recall the 60 and 100 watt models have different factory settings. I think it's in one of the service manuals.

 

[electric mayhem]

OK- I overlooked the "preamp" is class A. So if the power amp is class AB then surges could still be present at the power sockets and I'm glad I put the CC's on the screens.

The bias pot would have to be installed across the 50uF/75V- that's the R in question. I'm a little confused tho since the grid bias is set at the PI with either bias resistors (stock) or pot (modded)

That said the manual states -52V bias. I'll leave this one until I'm done with the amp and then power it on jumpering a 3k-5.6k range R and then dial in the -52V.

The manual also states the V1 plates should use 33k-100k R depending on a lop-sided wave form but I still need to look into non-fetron optimization.

 

[mark1406]

This is how my SOB is set up to deal with biasing the output valves.

As shown in the first shot, one resistor is replaced with a ten turn pot and it’s stuck in place with avionic grade silicon.

The second shot shows one ohm resistors going to ground which allows me to measure the current going through the valves. In the case of these two resistors I used a very accurate ohmmeter (without leads) and put one ohm resistors in the amp that were 1% accurate or better (overkill for sure.)

 

[electric mayhem]

This is how my SOB is set up to deal with biasing the output valves.

As shown in the first shot, one resistor is replaced with a ten turn pot and it’s stuck in place with avionic grade silicon.

The second shot shows one ohm resistors going to ground which allows me to measure the current going through the valves. In the case of these two resistors I used a very accurate ohmmeter (without leads) and put one ohm resistors in the amp that were 1% accurate or better (overkill for sure.)
View attachment 838

View attachment 839

I picked up the IIB's from Charlie Daniel's estate which included 2 coli's. He had bias pots installed on both at the PI which is where I expected they would be.

Is this the same function but just located on the PS board instead?

 

[electric mayhem]

A note on the PS board leads:

The original capacitor leads are used on the underside like rails. I've done more cut lead/solder on this amp than I usually do. When I replaced the PS filters I cut the leads and soldered to the top of the board. When soldering the new voltage drop R's at the 5.6k/10kohm junction the old cap lead pushed out.

I stuck a lead through the hole and soldered top and bottom but if I had to do this again I would have not clipped the filter leads, cleaned the holes and replaced the new cap leads through to the bottom "rail".

It's not like you have to worry about pulling a pad here. Any idea what these old red boards are made of?

The power output board used a rubber sheet underneath like the EQ boards have used so bridging shorts to ground doesn't seem likely.

 

[mark1406]

The bias circuit is on the power supply board of the SOB. The bias circuit is on this 1976 vintage power supply board too.
The SOB has a bit more realestate for the pot.

The board might be made of Garolite, it’s very popular with boutique amp builders.

 

[electric mayhem]

I should have added some bourns pots to the order I just made tho I'm not sure where I would mount one.

Speaking of pots does anyone have the code for the old 70's/80's cts pots? I did a search on here without luck and in internet-land only found the newer style code which I tried to apply below. My Bass shaft is broke and the slave is jacked.

Here's what I have and I don't see anything from '76:
Volume 1 JK1848A (pull boost) from '78
Volume 2 M174.... need dental mirror or pull it, (pull bright)
Master GH1047-1 from '83
Treble GH1043A
Mid GH1551 from '80
Bass GH1048 from '79

Slave M1278-1 from '83
Presence GH1048A from '83
Reverb 1050-1 from '83

I saw someone posted that a standard presence pot is 100k which would confirm 104=10k if we took the first 3 digits like a newer cts pot is marked.
e.g. V1=180k, treble=100k, mid=1.55M, bass =100k, reverb=1M
slave=100M?

form size / style (first letter(s)?), percent and taper still needs to be there and the 1-8 newer code doesn't jibe with a 4th digit being a zero like the reverb pot. 'A' could be audio taper and so could 1 for that matter.

I'll probably pull the bass pot tomorrow,test and disassemble. I know mesa rebuilds unavailable ones and I may have the right one but if I can find a donor shaft maybe I can reuse it.

The slave needs a smaller profile knob and shaft.