First Bassman Project Re-visited

[Platefire]

I converted a Bogen CHB50 years ago into a 5F6A. That was in my early days before I started building turret boards, so this was old style point to point hay stack/rat nest style wiring. The amp sounded good but always had a hum the bugged me. Recently fooling around the that Marshall JCM800 with a bad hum(that I didn't fix) got me thinking about my old first Bassman project since the circuits are similar.

So the guy came and got the JCM800 Tuesday and Wednesday I pulled out the old bassman head and started giving it the once over. Found several locations for some reason I tied the ground terms together in several terminal strips with a wire that I now removed. I found where I ran the NFB wire to the presence control directly under the filter caps and re-routed that. Seperated a lot of wiring that was touching that could potentially make noise.

Also I did something kind of unothodox--the input grid wire to first gain stage was super sensitive so I had an impulse to try something--in which I wrapped another piece of hook up wire wire around the grid input wire looping it its full length and tied one end of the wrap around wire to ground. It reduced the sensitivity and I guess it was kind of like a shielding. I left it on because it seemed to help!  

Guess what, after all that the Bogen/Bassman is running nice and quiet now. Sounds so good!    I needed a little victory after that tough JCM800 experiance. Platefire    

[tubenit]

the input grid wire to first gain stage was super sensitive

You are or aren't using shielded wiring for the input wiring with the grid resistor on the socket?

With respect, Tubenit

AND ......... have you thought about paralleling the V1 on the Bassman?  You can use a spdt center off to get different cathode values

[Platefire]

I haven't been using shielded wire on the input--even though I have some, never have used it.   Also the grid resistor is on a terminal strip. The thing I did on the bassman should be proof to me that real sheilded wire is a advantage for low noise! So what is the advantage of locating the grid resistor on the tube socket? I'm like you, I love a quiet amp.

Yes I have been considering doing something with that extra V1 gainstage and also putting in a MV. I have been real happy with the way the switched in gain stage works on the 5DC3 Deluxe amp you helped me with earlier this year. I never have tried parelleing an input gain stage. What effect does it have compared to switching in that same gain stage in series? I guess when I do that, I could put in a real piece of sheilded wire I will have a look at your attached schematic & pix, Thanks! Platefire

BTW-have you ever looked at the Pignose G40 approch to the bassman circuit. I have that amp and it works pretty good. The extra gain stage is used in conjuction with the pre-volume.  

 

[tubenit]

Platefire,

Paralleled triodes give about 30% gain increase with no increase in noise.

But ............ triodes in series are very cool too.

OK, this would give you a VERY versatile amp with only two switches.  Center off  spdt & a dpdt.

This is a variation of Hoffman's  "hot switch" for a Marshall.  However, this uses a 1M pot which I recommend. If fact, you may want to parallel a 1M resistor from the wiper to ground on a 1M pot if there is too much gain making it a 500k pot (sort of).  Hoffman's hotswitch uses a 470k/470k voltage divider and I found that did not give me enough control over the gain.

IF need be, you can use a mini-pot of 1M on the front or a 1M on the back panel.

I recommend the 100k/1.8k or a 100k/2.2k on the V1b. I also would not use a cathode cap larger than 4.7uf and would recommend a 2.2uf as a starting place. You might need to reduce that to a 1uf?

Be sure to use shielded wiring on any long runs from the dpdt if you do this mod. You will be creating a reasonably high gain amp.

IF you look at the Dumble or Trainwreck amps, you'll see they use smaller coupling caps in between OD gain stages. And they will use smaller cathode cap values on those OD gain stages.  

So the .01 off the V1-6 might even need to be reduced to .0047?  Your ears will need to be the judge.

I attached a SCH editable schematic for you so you can post what you finally do. Please let us know what you come up with.

Add shielded wiring. Used correctly, it will NOT hurt your tone and may improve it by filtering out some high end hash.  I like the grid
resistors to be right on the socket pins.


With respect, Tubenit

[tubenit]

OR ................. you can also add a PAB (preamp boost) that somewhat bypasses the tone stack for another boost in
gain.  I have this on all of my amps and use it quite a bit.  Super simple to do.  Use a 10M or 22M resistor on the spdt.

So you could either have a paralleled V1 (30% boost) with PAB

or

The HOffman "hotswitch" with a gain pot and the PAB.

Both good choices.

[echuta13]

Didn't you also do a Bassman on a Dukane chassis once upon a time?  I have a couple Dukane's floating around here somewhere...  Thinking I might need to convert them to something usefull @ some point.

[Platefire]

Thanks, tubenit

I'll will be weighing my options presented. One thing in mind though, the amp has presently got a bell like clean tone that I love and want to preserve. So I thinking switchable mods when turned off return the amp to it's original state.

Eshuta13-yes I do have a bassman Dukane. One thing I would say about the dukane is the bias supply voltage from the Dukane PT didn't supply enough voltage needed for the 5F6A bias, so I had to tap the secondary to the rectifier to build a bias circuit like on my finished schematic provided with the post in 2006.

On an added note, I made mention on a previous post about the Pignose G40 Schematic. On futher inspection of that schematic it appears very close to the JCM800 2204 when plugged into the high input adds the extra gain stage prior to the tone stack. Platefire 

[macula56]

how's that modded Hot Switch work tubenit? does it vary the gain or act like a mv?

[Platefire]

tubenit. One of the main mods I was interested in on this amp was a MV. Please look a my revision for MV on attached and see if this looks correct. Platefire

[sluckey]

Your type 4 MV is correct.

[tubenit]

how's that modded Hot Switch work tubenit? does it vary the gain or act like a mv?

It adds gain.

Tubenit

[Platefire]

Ok good on the (type 4?) MV. Regarding the last question on the modded hotswitch, I'm thinking that the volume pot would give you more control of the the added hotswitch(and V1a) gain when switch in as Doug's setup would be a fixed amount either on or off.

The way I plan to work these mods, and tell me if this is a wrong approch---to add the MV in first and test the sound/operation of the amp before moving to the cathode resistor/cap switching, hotswitch or/and PAB. Simply because I am wanting to maintain my present basic clean sound and I want to make sure I don't loose it at any point. So I hoping with the MV at 100%, I will still get my present clean tone. So if the last mod works/sounds good, accompolish the next mod and test again. 

On my Bogen CHB50 as far as MV real estate. I think I can put my pre-volume in my present input jack location and squeez the input jack in-between the new pre-vol location and end of face plate---about 2 1/8" space from the center of new pre-vol location and end. Platefire     

[tubenit]

The way I plan to work these mods, and tell me if this is a wrong approch---to add the MV in first and test the sound/operation of the amp before moving to the cathode resistor/cap switching, hotswitch or/and PAB. Simply because I am wanting to maintain my present basic clean sound and I want to make sure I don't loose it at any point. So I hoping with the MV at 100%, I will still get my present clean tone. So if the last mod works/sounds good, accompolish the next mod and test again. 

Platefire, since ALL those mods are switchable ........... you can always return back to the clean sound. You may or may not like the mods but they won't harm the clean tone.

Regarding real estate on the amp chassis.  I have used the mini-pots like one would find on a effects pedal just fine with no issues. So you could consider that as an option.

With respect, Tubenit

[sluckey]

Ok good on the (type 4?) MV.

Type 1, 2, 3, and 4 are just a naming convention that Ken Fischer (Trainwreck) used when writing about different MV circuits. The names kinda stuck but there's usually ???s whenever someone mentions a MV by Fischer's "type". Your particular MV is the same as JCM-800 2204, or type 4.

[Platefire]

tubenit-Guess your right on the switching, once they are off they are non-existant. Only thing on cathode switching my existing V1a is 22uf/1500 R default----and with your combination default/center off looks like Marshall 2204.  

slucky-Thanks for the MV explination. Guess I need to read up on that. Makes me wonder what # my Accomplice is with a double 1M post PI.  Thanks, Platefire

[sluckey]

Accomplice Jr. uses type 2. Here's all you'll ever want to know about those 4 master volumes....

http://blueguitar.org/new/schem/trainwreck/the_trainwreck_pages.pdf

[Platefire]

Thanks, I downloaded that trainwreck file. Looks like interesting bed time reading. It's kinda strange going back and reworking some of my original projects. Some of the stuff I did back then made no sense at all and they really do need to be corrected. I'm looking forward to the improvements and new versatility that's coming. Thanks, Platefire

[Platefire]

I went over the Trainwreck Pages last night and I may of missed it but I didn't see anything where they mentioned the method of biasing using 1 ohm resistors to power tube cathode to gound with test points jacks to outside of amp. If you install the locking bias pots for outside adjustment--that is the most convenient way ever for "fixed bias". Of course I've mostly built Fender type circuits and don't know much about Marshalls. Seems Trainwreck is more Marshall oriented. Platefire

BTW-It's amazing how much difference a bias adjustment makes on amp performance. On the Bogen/Bassman and the Peavey/DRR were responding cold and stiff with a ratteling in the speaker to the point I thought my new 10" Ragin Cajun had gone bad. I checked the bias and both had drifted cold. I rebiased and gosh, the reponse/tone became heavenly with no rattles. What a differnce a correct bias adjustment makes! 

[Platefire]

    I did the MV mod tonight. Went pretty smooth but when I finished and fired it up I got a loud buzzz. Had to look at the circuit a while and finely saw it, the grid input wire had come loose from the input jack--so I soldered that back and it was good to go. Jammed on it a while and it was nice and quiet as far as operational noise. Played around with the volume settings and pretty much found my sweet spot. Works real good. I think I'm going to have to order some parts before I proceed with the other mods. Platefire

[Platefire]

Tubenit

Now that I got the MV in and working well, I think I'm going with Hot Switch with MV and the PAB. I may add some V1a cathode switching if I have room--studying available real estate right now for the other mods. On cathode switching do you have a combination that I can get a setting 25uf/1500 Ohms(close to fender) and a Marshall setting as an option? or does the combination shown on the revised schematic already have that?

Isn't that what we did when you helped me with the RM Tweed deluxe with the cathode switching setup for V1b of attahed layout? Platefire

 

[tubenit]

Platefire,

Use a dpdt center off

[Platefire]

Got Ya! Thanks

Man! Your a great help!  

But you did mean SPDT instead of DPDT didn't you?


Yes, SPDT center off.  Tubenit

[Platefire]

Ok, here is what this rat nest I was refering to looks like! Bare in mind this is one of my early conversion projects and I'm now trying to apply my more recent amp knowledge to make improvments without scraping the whole thing. I will say it sounds better than it looks
 Here is a few comments or questions:

Pix#1-General Bogen CHB-50/5F6A in rat nest form-using Bogen CHB-50 Power supply/doubler design

Pix#2-10 ohm/10W resistor in power supply slightly burnt with blisters! I think I need to order a replacement with my other mod parts, wouldn't you agree???

Pix#6-Power Supply & Bias Circuit

Pix#8-Preamp section where I will be installing cathode switching and Hot switch with MV. The switches will have to go on back of chassis--requiring long grid runs!!! I have existing preamp out jack and V2a cathode bypass cap switch that will have to be removed for new mods.

Now that you can see my sit-sea-a-tion, well what cha think? I'm making a material list for all my mods and want to make sure I get everything in one shot. Platefire

 

[tubenit]

Pix#2-10 ohm/10W resistor in power supply slightly burnt with blisters! I think I need to order a replacement with my other mod parts, wouldn't you agree???

Yes.

I think the long runs will work out OK. You can always use shielded wiring on the hotswitch DPDT which is what I would do.

with respect, Tubenit

[Platefire]

Well I'm trying to think this one out fully before I pull the trigger on a parts order. I think I might could install the cathode switch and PAB switch between existing knobs on the front panel that would be a lot more handy than on the back. With the hot switch needing the volume control close by, the back fourty is most likely location for that.

Question: is the PAB switch a DPST with no center off? I think that I could locate that switch right between the treble and bass pots on front panel. Also on the schematic showing the PAB switch it shows the wire from center term of the PAB switch connected to the the bass pot term that the .02 cap is connected to and also the middle term of the bass pot. Is that correct???

Thanks, Platefire

[tubenit]

is the PAB switch a DPST with no center off?

It is a SPDT switch or you can use a DPDT switch.

Look at the SCH files for the TOS schematic and layout which shows the tone stack and PAB wiring.

with respect, Tubenit

[Platefire]

Tubenit. Well I ordered parts from Doug today. Was able to get most what I needed with the exception of the 10 Ohm/10 W wire wound resistor. I located one at AES, so I guess I'll get it from them. Also I did look at the TOS sch related to PAB but what I saw to didn't directly apply to installing it in a 5F6A. Anyway I revised the schematic you provided to what seemed right to me that may be all wrong!  Xo have a look. Platefire

[tubenit]

Platefire,

The way I drew the PAB on Reply #4 is how I have my TOS wired.  There are numerous ways to wire a PAB that work.

with respect, Tubenit

[Platefire]

Thanks tubenit, and please bear with me. I know all information you put out is tried and proven before you share it. What is confusing me on your schematic on reply #4 is that I see a wire running through terminal A(normally the pot input) on the bass pot which on the fender 5F6A schematic is a "no-connection". So your schematic don't show a dot(connection) at that point of the wire, so you may have intended it for a "no connection" without the dot(connection). I just need to clairify that connection on the input of the bass term rather it would still be a "no connection term" or "connected". Forgive me for going back over this but my brain won't let me rest to proceed until I can get some understanding on something I'm not comprehending---even though coming from you, it would work anyway--regardless of my understanding. Thanks, Platefire  

[sluckey]

The bottom of the treble pot, top and wiper of the bass pot and .02 cap all tie together. I know that Fender did not connect the wiper to the top lug of the bass pot on the 5F6A, but doing so is the preferred way. Nothing is changed as far as circuit operation but now you don't lose the tonestack if the bass pot wiper loses contact to the pot's carbon track.

[Platefire]

Ok, thanks for you guys patients with me and your help. Just wanted to be sure about it. I sat down last night and sketched out a layout plan of the three mod additions I want to do and that one was the only one I wasn't sure about(the PAB). So now I feel pretty much prepared. It just bothers me on the hot switch I got four rather long runs back and forth to the switch & pot to be mounted on the back panel. I plan to make all those runs with sheilded cable, so hopefully that will hold down any noise issues. The amp sounds so much better now with the mistakes I found, corrected and the new MV, that I almost want to leave well enough alone but----I feel if I am going to expand my skills I need to push through my fear and experment. It's a little scarry but adventurous! 
Guess thats' why I like it. Platefire

[Platefire]

Can't believe it! I actually found that 10 Ohm/10 watt wire wound resistor that I need to replace in my power supply at Radio Shack.    1 up for RS!

[Platefire]

Found another couple of mistakes from the original build again yesterday. I doulble checked the OT hookup referencing Doug's diagram for his part #18343 Bassman OT. I've got the green/yellow 4 Ohm tap hooked up to the speaker jack and all my speakers are 8 Ohm hookups. On top of that I was using the 27k Ohm NFB resistor as used in fender original for 4-10 cab. I changed to the green 8 Ohm tap and to 56K NFB resistor that I think is adjusted for the 8 Ohm tap.

I'm trying to remedy and get to the bottom of the raspy distortion I'm hearing when the amp is cranked up. I'm beginning to feel that's something I need to solve before I do these high gain mods.
The distortion sounds pretty good/full at low power amp volumes and high preamp volumes but as I turn the MV up and get more power amp into it the distortion is not fat and full but has a high end rattle to it even when adjusting more bottom into it with the tone stack. I've been looking at everything trying to figure out why and remedy it. I think that last corretion helped but didn't completly correct it from last taste test I did. Platefire

[Platefire]

I installed the PAB mod tonight and something is bad wrong. First of all, my tone stack with the PAB off is changed in a very bad way. It's all bright and no bottom no matter how you adjust it. The tone controls are effecting the tone as you turn the knobs--just very brittle bright tone no matter how you set it. Then when I engage the PAB it is a boost all right but again brittle bright tone with no bottom. Attached pdf is a sketch of the current hookup(changed back per edit below). Platefire  

EDIT: I rewired my tone stack back like it was previouly with mechanical connections (not solder) and got my sound(bottom) back. Whew! That's a relief! I think to maintain the sound I like with my current tone stack hook up I would have to hook up the PAB like I originally determined(attached sch). I hope the PAB when engaged don't go super bright like it did in the other hookup or it will be un-usable for me.

[Platefire]

OK! I discovered why my tone stack wasn't sounding right when the PAB was switched off in the way sluckey directed me to wire it. I failed to install a jumper between the input term and the wiper term on the bass pot where the in series 56K and .02 coupling cap attach to the previous no/connection term. I attached a pdf of my corrected sketch showing the jumper I left off---this sketch is drawn as if you were looking at the back of the pots. The tone stack works perfectly normal with this hookup. My Bad!  

Problem is with the PAB switched on/engaged, the boosted sound is a hollowed out bright tone that is not pleasing. I like my drive and distortion fat as I think most folks do. This is an unusable tone for me. The circuitry the signal is passing through when the tone stack is cut off is working like bright switch boost.

Is this un-workable for a good thick boost on this paticular tone stack? If it is workable, please show me what I'm missing. Platefire

[tubenit]

Platefire,

I think the reason your PAB sounds thin is you are using a 250p treble cap. I typically use a 390p or a 500p & the larger cap will make a difference!

The PAB is not particularly "thin" on any of my amps.  Having said that, the mid boost is a thicker tone and also boosts quite well.  In fact on my Tweed Overdrive, the mid boost has more of a boost then my PAB does.

Note the mid-boost has either a .002 to treble ........ OR two caps in series for around 326p (I think).

You could use a single DPDT and wire each "half" to simultaneously switch on the PAB with the mid boost. I seriously doubt that
you would not have enough of a fat tone with that.
 

OR ........ just wire in a midboost & skip the PAB.

With respect, Tubenit

I also included another PAB design that I have used that simply adds resistance between mid pot and ground

[ac427v]

Platefire,
When I did my bassman, I messed around with the feedback resistor. For my 4 ohm output tap, I used a 56K resistor. I think that would translate into 100K for your 8 ohm tap. The larger resistor reduced the feedback signal which seemed to smooth the distortion.

[sluckey]

Your mid pot is connected to the wrong side of the presence pot. It should connect to the grounded lug. See your schematic and this pic...

[Platefire]

sluckey, I think I've got my presence control wired correctly. The error is probably in my sketch. I did the sketch about 3:00 AM and was frustrated because I couldn't get the PAB mod to work right. The presnce control has been working fine and I haven't done any re-wireing on it lately. However I will check it next time I get into the amp to be sure. I had to lay my amp mod activities aside until after Christmas because the wife was getting pretty ticked because I get pretty obsessed with this stuff and tend to forget my other responsibilities when amping. Kind of like a mad scientist with a crazed look in his eye. Lot of Christmas stuff I need to do to prepare for Christmas. I will be dropping by the forum from time to time, but no solder flying.

tubenit, Thanks for the alternative plans on this boost switch. I'll have a look and study those. I figured it was that 250p cap killing me on the PAB. I think it's actually 270p in there. I will tackel it again after Christmas.

ac427v, It was a 27k NFB resistor in there and I changed it to a 56K the other day--so I should be good on that.


Thank you very much and I hope you all have a Merry Christmas
  

[sluckey]

There's nothing wrong with the presence pot wiring. The problem is the mid pot wiring. Referring to your drawing (either of the scanned hand drawings), you show the mid pot connected to the presence pot right lug. That's wrong! The mid pot left lug must connect to ground. If you want to use the grounded left lug of the presence pot that's OK, but you cannot connect the mid pot to the presence pot right lug.

[Platefire]

  Thanks sluckey for pointing that mid pot wiring out and if I hadn't already screwed the chassis back in the housing, I would have gone and looked to check the wiring already. When I get back working on it, that will be the first thing I check because no changes related to the tone stack will work right if it's wired wrong.

You see I don't have a shop area to leave my work in progress all torn apart and laid out, I have to clean it up and put it away until next time. I envey all you guys with nice shop areas. On the other hand, I'm thankful for what I do have. Platefire