68 Bassman Frame Off

[p2pAmps]

Here is a little Bassman for a customer I am doing up.  I ordered a Fiber Eyelet board from Doug and I have to say it's really really nice.  This old 68 Bassman was pretty bad when I got it.  Most everything was shot so it's getting the full treatment.  Left channel is a Marshall-esque (2204) mod and the center channel is an AA864 circuit...  Should be ready to fire in a few days and get her burned in. 

Thank You Doug for always shipping my parts so fast and accurate...

[eleventeen]

Looks excellent. I will tell you, however, that you are committing a theroetical grounding error with those parts connected to chassis via that lug between the little tubes. Why?/How? Because you are utilizing the chassis as a signal ground and you are doing so on the first preamp tubes which are more/most susceptible to hum since they are so early in the amplification chain. All kinds of weird nonsense can go on with grounding in a steel (magnetically sensitive) chassis which does not conduct as well as copper wire. It is not predictable, all you can do is to try to follow "best practices" as best you can. This would not be that.


You may have no problems with it at all, then again.....


The simple solution would be to unbolt that lug from the chassis, get it up on an insulated standoff, and use a nylon screw to hoist it off the chassis. Then, connect it via a decent sized wire to whatever you're going to use as your signal ground on the parts board itself. You shouldn't have to unsolder or rebuild anything on that lug, just lift it up 3/8" or 1/2", your leads look long enough as they are. 


Looks like you steel-wooled the OT. Did you *paint* the PT?

[p2pAmps]

Looks excellent. I will tell you, however, that you are committing a theroetical grounding error with those parts connected to chassis via that lug between the little tubes. Why?/How? Because you are utilizing the chassis as a signal ground and you are doing so on the first preamp tubes which are more/most susceptible to hum since they are so early in the amplification chain. All kinds of weird nonsense can go on with grounding in a steel (magnetically sensitive) chassis which does not conduct as well as copper wire. It is not predictable, all you can do is to try to follow "best practices" as best you can. This would not be that.


You may have no problems with it at all, then again.....


The simple solution would be to unbolt that lug from the chassis, get it up on an insulated standoff, and use a nylon screw to hoist it off the chassis. Then, connect it via a decent sized wire to whatever you're going to use as your signal ground on the parts board itself. You shouldn't have to unsolder or rebuild anything on that lug, just lift it up 3/8" or 1/2", your leads look long enough as they are. 


Looks like you steel-wooled the OT. Did you *paint* the PT?

Thanks.  So I hear you loud and clear on the grounding subject but I do have to tell you, I have done this mod at least 50 times the same way and these Bassmans are incredibly quite.  To the point of turning everything wide open and only hearing the slightest of sound.  My scope shows pretty quite too. 

I am not one to ever question the experts on this forum as I am a rookie and I know it.  I would be interested to hear why my Bassmans are so quite with this potential ground issue on the Left channel Marshall Mod...

Thanks and looking forward to hearing back...

Mike

PS, that PT was already replaced and yes I did shine up the OT...  Choke is orig too. 

[HotBluePlates]

... This old 68 Bassman ...  Left channel is a Marshall-esque (2204) mod and the center channel is an AA864 circuit... 

I did exactly that in a AB165 Bassman I used to have. One channel became a 5F6-A/Marshall channel (it also became much louder than the straight Fender channel).

Because you're changing the feedback injection point at the phase inverter, your OT plate wires will need to be swapped from what they were stock (or they stay and the secondary wires get swapped). Or, basically assume it's like a new build and you have a 50/50 shot of getting it wrong... 

[Ed_Chambley]

Looks excellent. I will tell you, however, that you are committing a theroetical grounding error with those parts connected to chassis via that lug between the little tubes. Why?/How? Because you are utilizing the chassis as a signal ground and you are doing so on the first preamp tubes which are more/most susceptible to hum since they are so early in the amplification chain. All kinds of weird nonsense can go on with grounding in a steel (magnetically sensitive) chassis which does not conduct as well as copper wire. It is not predictable, all you can do is to try to follow "best practices" as best you can. This would not be that.


You may have no problems with it at all, then again.....


The simple solution would be to unbolt that lug from the chassis, get it up on an insulated standoff, and use a nylon screw to hoist it off the chassis. Then, connect it via a decent sized wire to whatever you're going to use as your signal ground on the parts board itself. You shouldn't have to unsolder or rebuild anything on that lug, just lift it up 3/8" or 1/2", your leads look long enough as they are. 


Looks like you steel-wooled the OT. Did you *paint* the PT?

Thanks.  So I hear you loud and clear on the grounding subject but I do have to tell you, I have done this mod at least 50 times the same way and these Bassmans are incredibly quite.  To the point of turning everything wide open and only hearing the slightest of sound.  My scope shows pretty quite too. 

I am not one to ever question the experts on this forum as I am a rookie and I know it.  I would be interested to hear why my Bassmans are so quite with this potential ground issue on the Left channel Marshall Mod...

Thanks and looking forward to hearing back...

Mike

PS, that PT was already replaced and yes I did shine up the OT...  Choke is orig too.

I think the grounding is fine and have questioned this myself.  I just got a new scope and the first thing I tested it with was the grounding of an EF86 in V1 position.  I have wondered if it were best to ground the tube like you have done or ground it at my common preamp ground.  I have 2 amps with V1 being an EF86.  The Vox Ac15 is grounded at the preamp and it is about 6" of wire.  The amp is dead quiet, but I have another build which is a higher gain amp with an EF86 grounded at the tube socket and it has more noise.  I do not like noise so I tested to see if changing the ground would help.  The scope actually showed more noise when I changed the ground to the preamp.  Turns out I needed a better chassis shield and another shielded wire.  It is quiet now .

Don't know for sure, but my guess is with the shorter leads and not a lot of interference in the chassis area the "flying" components have less opportunity to generate noise.

When you run your input wires, are you using shielded wire? 

[p2pAmps]

Looks excellent. I will tell you, however, that you are committing a theroetical grounding error with those parts connected to chassis via that lug between the little tubes. Why?/How? Because you are utilizing the chassis as a signal ground and you are doing so on the first preamp tubes which are more/most susceptible to hum since they are so early in the amplification chain. All kinds of weird nonsense can go on with grounding in a steel (magnetically sensitive) chassis which does not conduct as well as copper wire. It is not predictable, all you can do is to try to follow "best practices" as best you can. This would not be that.


You may have no problems with it at all, then again.....


The simple solution would be to unbolt that lug from the chassis, get it up on an insulated standoff, and use a nylon screw to hoist it off the chassis. Then, connect it via a decent sized wire to whatever you're going to use as your signal ground on the parts board itself. You shouldn't have to unsolder or rebuild anything on that lug, just lift it up 3/8" or 1/2", your leads look long enough as they are. 


Looks like you steel-wooled the OT. Did you *paint* the PT?

Thanks.  So I hear you loud and clear on the grounding subject but I do have to tell you, I have done this mod at least 50 times the same way and these Bassmans are incredibly quite.  To the point of turning everything wide open and only hearing the slightest of sound.  My scope shows pretty quite too. 

I am not one to ever question the experts on this forum as I am a rookie and I know it.  I would be interested to hear why my Bassmans are so quite with this potential ground issue on the Left channel Marshall Mod...

Thanks and looking forward to hearing back...

Mike

PS, that PT was already replaced and yes I did shine up the OT...  Choke is orig too.

I think the grounding is fine and have questioned this myself.  I just got a new scope and the first thing I tested it with was the grounding of an EF86 in V1 position.  I have wondered if it were best to ground the tube like you have done or ground it at my common preamp ground.  I have 2 amps with V1 being an EF86.  The Vox Ac15 is grounded at the preamp and it is about 6" of wire.  The amp is dead quiet, but I have another build which is a higher gain amp with an EF86 grounded at the tube socket and it has more noise.  I do not like noise so I tested to see if changing the ground would help.  The scope actually showed more noise when I changed the ground to the preamp.  Turns out I needed a better chassis shield and another shielded wire.  It is quiet now .

Don't know for sure, but my guess is with the shorter leads and not a lot of interference in the chassis area the "flying" components have less opportunity to generate noise.

When you run your input wires, are you using shielded wire?

Agreed Ed.  And yes I always use shielded input cable.  I used to use Mogami and have started using the smaller stuff Doug sells and it is just fine. 

[p2pAmps]

Looks excellent. I will tell you, however, that you are committing a theroetical grounding error with those parts connected to chassis via that lug between the little tubes. Why?/How? Because you are utilizing the chassis as a signal ground and you are doing so on the first preamp tubes which are more/most susceptible to hum since they are so early in the amplification chain. All kinds of weird nonsense can go on with grounding in a steel (magnetically sensitive) chassis which does not conduct as well as copper wire. It is not predictable, all you can do is to try to follow "best practices" as best you can. This would not be that.


You may have no problems with it at all, then again.....


The simple solution would be to unbolt that lug from the chassis, get it up on an insulated standoff, and use a nylon screw to hoist it off the chassis. Then, connect it via a decent sized wire to whatever you're going to use as your signal ground on the parts board itself. You shouldn't have to unsolder or rebuild anything on that lug, just lift it up 3/8" or 1/2", your leads look long enough as they are. 


Looks like you steel-wooled the OT. Did you *paint* the PT?

Well I finished the Bassman and I did leave the ground between V1-2 in place.  The amp is deal quite wide open.  I wouldn't even mention this but I have done this mod for many guys without ever having a ground or noise problem... 

Cheers...

Mike

BTW Doug, I love the fiberglass eyelet board!

[sluckey]

Nice clean look.

[p2pAmps]

Nice clean look.

Thank you sir....  Good thing I'm not trying to pay the bills with amp rebuilds LOL

[Ed_Chambley]

 

[jojokeo]

I agree on things looking very nice but there's one thing missing? Where's the Dykem? (just mesing with you Mike) 
 
Just curious, what "Marshall mod" on the first channel did you use exactly? HBP's 5F6-A reference seems more Fendery than Marshally?

[p2pAmps]

I agree on things looking very nice but there's one thing missing? Where's the Dykem? (just mesing with you Mike) 

You're right!  I slipped up with the Dykem LOL

I will PM you on the Marshall mod

[HotBluePlates]

... HBP's 5F6-A reference seems more Fendery than Marshally?

The 5F6-A is identical to the first Marshall's. By using "5F6-A" I really meant Gain-Volume-Gain-Cathode Follower-Tonestack. You can alter cathode components in the gain stage to suit your single preferred Marshall preamp channel variant.