Help wanted Larry grounding a Caswell #39 build

[ivan H]

Hi all, I previously used Larry's grounding scheme on a 50 watt build that is quiet as. I'm waiting on a chassis & PT to arrive so I can start my next project, a 50 watt switchable #39/36 type amp & am wondering how to best implement Larry's grounding scheme into this. Basically it will be a 2204 with an added triode between V1 & V2, with a 10k decoupling resistor & electrolytic fed off the HT node at the cathode follower that will be in parallel with the rest of the pre amp supply (as Tim Caswell recommends). I plan on using a gain control after each of the first 2 stages & a pre PI master volume. Any help incorporating Larry's grounding scheme into this build will be much appreciated. Cheers

[ivan H]

Like, please correct me if this is wrong, but I'm thinking do just like normal with the following differences. As it'll only have 2 inputs, 1 above the other, move the 2nd lug a little closer to the 1st & add another between the 2nd & 3rd, then instead of grounding both gain pots at the 2nd lug, ground only gain 1 there & ground gain 2 at the added lug along with the added triodes cathode & the added electrolytic capacitor. The master volume would be grounded at what was the 3rd (now 4th) lug along with the treble pot, presence pot, output common etc. Does this sound right or have I got it all wrong. Thanks in advance for any help. Cheers

[ivan H]

Ok, 6burgh has answered this question for me in another thread, so I thought I'd also post it here, just in case anyone else needed to know. The added stage grounds atposition 2, the same place as the volumes, V2 & the dual can pre amp filter. The Pre PI Master, being after the tone stack, grounds at position 3. Cheers

[neikeel]

Yes I need to separate my master off the shared node on the bus rail (I have more hum than I want on mine!).

Interestingly on my version 1 I had an extra 10k dropper (and 32uF filter can) for the added stage but I found it dropped the volume for that stage but did add a little gain (seemed more fuzzy to me).

[neikeel]

As an update to Ivan H.

I had hum and looked back inside and saw that I had grounded the heater centre tap to the spare mounting bolt on the PI filter cap .

Disconnected it extended the tap wire round the front of the chassis to the input bus = silence! No hum - much happiness

[ivan H]

Hi Neil, thanks for the heads up. I have the heater CT wired to position 1 (running under the pots) using solid 18# cloth covered heater wire that I also used to wire the power tube heaters, running right around the outside of the chassis. The pre heater are wired with the normal 22# red & black wire. I had read of your finding with the extra 10k decoupler/electrolytic so at the moment have it without, tho I do have an axial 22uf B&C cap that I can mount on the board along with a decoupler so I can try both ways. Tim Caswell recomends adding these in parralell to the rest of the pre amp supply (off the node at the cathode follower) rather than in series with,,, this I imagine would take care of the voltage drop. For the board I got a blank piece of Brian Haberman's nice board material & populated it with turrets myself & have added turrets needed to do this, (plus a few others for tweaking purposes). I wired the board "on top" so its easier to modify while experimenting. The last package from Valvestorm arrived at the end of the week so I'll be doing some serious soldering today & tomorrow. I'll get pics of the build & see if I can get them posted when finished. Cheers

[neikeel]

Picture is here:

http://s76.photobucket.com/user/neikeel ... 5.jpg.html

Sorry the site will not allow big pics but the link will blow up, sorry the soldering is not as good as usual but it has been back and forth a few times! You can see where the other 10k dropper and cap was on the extra board.

[ivan H]

Hi Neil, thanks for the pic. I cant make out much detail as I'm veiwing on a phone, I'll get to a computer for a better look. So u had the 10k dropper & cap in parralell,,, I'm glad I left as it is then. Obviously no problems arise from having 2 in phase & 1 out of phase stages fed off the 1 node, I noticed Ceriatone does it this way as well. Quick question, r u using a 250pf or 500pf treble cap in the tone stack? The two 10k stages I'm thinking of leaving unbypassed & using a dual relay to switch in the .1uf bypass cap & also the 150k grid leak pulldown resistor, & another relay to switch in another 10k/.22uf in paralell with the second 10k cathode resistor, tho I'm open to suggestion on these. Actually, any input at all is very appreciated. Cheers

[neikeel]

Hi Neil, thanks for the pic. I cant make out much detail as I'm veiwing on a phone, I'll get to a computer for a better look. So u had the 10k dropper & cap in parralell,,, I'm glad I left as it is then. Obviously no problems arise from having 2 in phase & 1 out of phase stages fed off the 1 node, I noticed Ceriatone does it this way as well. Quick question, r u using a 250pf or 500pf treble cap in the tone stack? The two 10k stages I'm thinking of leaving unbypassed & using a dual relay to switch in the .1uf bypass cap & also the 150k grid leak pulldown resistor, & another relay to switch in another 10k/.22uf in paralell with the second 10k cathode resistor, tho I'm open to suggestion on these. Actually, any input at all is very appreciated. Cheers

Your plans sound pretty good, I went for simplicity, I could not get the amp to be stable without the 150k, I tried grid stoppers on the socket too. I left a 500pF cap across the plates of the third (added) stage.

Spent some more time with it yesterday, footswitching and trying different guitars (strat, Les Paul and my usual Martyn Booth). I like having the stock plexi option, with the PPIMV and prePIMV maxed it is stock 68SL, there is a slight intereaction with the gain knob (even though it is the unused channel in this situation). If you turn it up there is a bit more brightness when using the plexi channel, so there is a little bleed between channels.

I found a sweet spot to footswitch easily.

[ivan H]

Sounds like a versatile amp Neil. Did u use an unused triode to create a plexi channel connected via mixer resistors? With a 50 watt chassis space is at a premium so I'm just doing a #39/#36 type thing, tho with the 150k across the pot I can use the one relay to switch the cathode bypasses in & the other to switch the added stage in & out. I'm planning on using this amp in conjuction with my last 50 watt, switchable rectifier/main filter amp & use an A/B switch to switch between them. I've also added a switchable 250pf/500pf treble cap & switchable .68uf/unbypassed/25uf over the 820ohm cathode resistor before the cathode follower on this amp, & will also add a resonance control. I'm still undecided on the switchable added gain stage,,, maybe leave it off. Thanks again for your assistance, I appreciate very much. Cheers

[neikeel]

Sounds like a versatile amp Neil.

I think so.
You can plug into the usual hi channel 1, max the PPIMV and the pre and you are totaly stock

Did u use an unused triode to create a plexi channel connected via mixer resistors?

Yes - the original V1b 0.68uF/2k7 unchanged.

Only difference is putting a 10k (unbypassed) where the old 330/820 were and then switching this in and out only on the 2203 channel.
The 2203 preamp is totally separate on the added board. The signal from this is injected through the normal channel mixer (unbypassed). The 2203 with the specs developed/tweaked from the AFD thread is plenty bright enough and does not need the extra 470pF bypass cap on the 'normal' channel 470k resistor (but as I mentioned above there is some interaction but, IMO, in a positive way. The push-pull interupts the signal from the 2203 stage and inserts the extra stage. I cannot footswitch the extra stage but can between plexi and 2204 with some volume knob twiddling to optimise both if I want to do it on the fly.

With a 50 watt chassis space is at a premium so I'm just doing a #39/#36 type thing, tho with the 150k across the pot I can use the one relay to switch the cathode bypasses in & the other to switch the added stage in & out

Yes an issue. I am going to build a 50w with a stock bass and a stock lead preamp in the same way so I can have footswitchable shared or split cathode preamps based on a 69 Master PA I have but in special chassis to fit in a smallbox cab. Waiting on the chassis at the moment, but most of the build will be a transplant rather than fresh build.

Look forward to seeing your build and how you get over the problems (how did you get the valve/diode rectifier going in the end?).

[ivan H]

Hi Neil, the switchable rectifier/filter was quite easy to sort. I ordered 1 of the 3PDT switches from Valvestorm (hi quality switch) & while waiting for it to arrive I read up on different switching methods people have used. Feeding the AC to both recs in paralell & then using 1 pole of the switch to select the DC output, another pole to select hi or lo filtering & the 3rd pole to add the bias dropping resistor needed for correct bias in tube rec mode I didnt like as I think it is inviting the GZ34 to suffer flash-over when not in use, plus I dont like putting DC on switches if I can avoid it. Shakti's method of switching rec/filter/bias dropper seemed best to me, feeding the AC to the center terminals of 2 poles of the switch with 1 side of both poles going to the GZ34, the other side going to the SS rec. The bias dropper (AC) is placed between the center teminal & the Solid State side of 1 pole of the switch, which is then linked to the bias circuit on the board. When in tube rec mode the AC is fed to the bias circuit thru this resistor, & when switched to SS rec mode the switch bridges this resistor, taking it out of the circuit. The 3rd pole on then switches the added filter capacitance in while switched to SS mode. The only slight problem that I could see was that Shakti had the output of both rectifiers hooked to the filter cap that was common to both modes, or always in circuit. With this arrangment I figured that when switched to Tube rec mode, the full wave diode rectifier is still being fed almost the full AC voltage from 1 half of the secondry thru the bias dropper on the switch. This I figured would (a) increase the voltage that the tube rec alone should supply & (b) reduce the "sag" effect of tube rectifier, one of its most desireable features. When I installed it, to test my theory I hooked the output of the GZ34 to the 1/2 of the dual can cap that is always in circuit, & the output of the doides to the 1/2 of the can cap that is switched in while in SS rec mode. I then powered it up with no tubes intstalled, with the switch set to tube rectifier. I then measured the voltage on the cap that the diodes are connected to & sure enough, there was a large DC voltage present. I cant remember the exact figure off the top of my head but, as the cap was floating with no load or current drain at all, the voltage was high enough that I was worried about it exceeding the caps working voltage rating. To remedy this I put two 680k 1 watt resistors in paralell from the caps positive terminal to earth, meaning I had a 2 watt 340k bleeder resistor across the cap. Now when in tube rec mode the unused 1/2 of the cap sits steady at 385v. To calculate the required bias dropper needed I temporarily wired a 25k linear pot across the switch instead of the resistor. I baised it up in SS rec mode, then, with the pot wired to the switch set to minimum so it supplied the highest possible AC voltage, I baised it up in tube rec mode using the pot, with the bias trimmer left where it had been set for SS rec mode. Once the bias was set in tube rec mode I powered it down & unsoldered the pot from the switch & measured the resistance which was almost exactly 15k. I then measured a handfull of 15k 1 watt resistors to find the one with the closest value to that that the pot showed & soldered it in place. Bias is now just a tad under 70% max plate dissipation in both modes. The system works very well. To save the GZ34 from the strain of filter capacitor inrush current demands I power the amp up with the switch set to SS mode & allow a couple of minutes for the filters to charge, then flip the switch thru the center standby position to tube rec mode (or leave in SS mode). I have 50uf main filter in tube rec mode & 100uf in SS rec mode, all others are 32uf. Other spec are V1 cathodes 820/25uf (instead of 330uf), 820/.68uf, 470k/470k/560pf mixer, V2a 820/.68uf, 560pf/33k tone stack, .022uf output couplers, 5k/.1uf presence & 47k/8 ohm tap NFB. The amp went thru a few formats over 6 or more months, including switchable cascaded 2k7/.68uf, 2k7/.68uf before I settled on this current spec & added the dual rec thing. I almost always have the inputs jumpered & it runs thru a 1960A cab loaded diagonally with 2 greenbacks & 2 vintage 30's @ 16 ohms. Sounds great. Cheers

[neikeel]

Sounds well worked out!
Need to sit down with a pen an paper before I try to implement it!

[ivan H]

Hi Neil, I had to think about it a bit at first, but I'll give u a quick lowdown so its easier to visualize. I chose to have the toggle down to select the tube rec (lower voltage & main filter) & up for SS (higher voltage & main filter), so with the chassis out of the cab & upside down the connections on the 3PDT switch are; bottom row, left to right. Left & center to GZ34 anodes, right terminal not connected. Center row left to right, left & center to PT secondries, right terminal to the filter that is always in circuit. Top row, left to right, left & center each have one half of the full wave diode rectifier attached to them. The other end of the rectifier can connect either to the right terminal of the top row or to the filter that is only used in SS mode. Either way, this filter is also connected to the right terminal of the top row. The bias dropping resistor is connected to the left terminal of both the center row & top row of terminals. Dont forget the 340k 2 watt bleeder resistor across the filter that is switched in during SS mode. I am very happy with how it turned out. BTW, I was just reading ur #39 spec in the SIR 100SL thread. I personally dont have the need to exactly recreate Slash's AFD tone. Rather, I really like Tim Caswell's idea of keeping the basic 2203/4 tonality while adding a little extra gain and sizzle to the higher strings. Now my arithmatic is pretty bad but the way I figured it, if one wanted to bypass a 10k cathode resistor & keep the frequency responce similar to that of a 2k7/.68uf stage, then u would use a .2 or .22uf as this puts the 6db roll of point just slightly lower than that of the 2k7/.68uf stage whereas a 10k/.1uf would put it a fair bit higher. My math could be completely wrong but either way I think I'll follow your lead of trying a .2uf over the 1st 10k cathode. I am almost ready to install the board & the only components not on it are 10k bypass caps & plate bypass caps. I used some circuit board cleaner to remove the resin spatter last night & will do a few small jobs inside the chassis each evening so it right to install the board this weekend. Cheers

[neikeel]

Hi Neil, I had to think about it a bit at first, but I'll give u a quick lowdown so its easier to visualize. I chose to have the toggle down to select the tube rec (lower voltage & main filter) & up for SS (higher voltage & main filter), so with the chassis out of the cab & upside down the connections on the 3PDT switch are; bottom row, left to right. Left & center to GZ34 anodes, right terminal not connected. Center row left to right, left & center to PT secondries, right terminal to the filter that is always in circuit. Top row, left to right, left & center each have one half of the full wave diode rectifier attached to them. The other end of the rectifier can connect either to the right terminal of the top row or to the filter that is only used in SS mode. Either way, this filter is also connected to the right terminal of the top row. The bias dropping resistor is connected to the left terminal of both the center row & top row of terminals. Dont forget the 340k 2 watt bleeder resistor across the filter that is switched in during SS mode. I am very happy with how it turned out. BTW, I was just reading ur #39 spec in the SIR 100SL thread. I personally dont have the need to exactly recreate Slash's AFD tone. Rather, I really like Tim Caswell's idea of keeping the basic 2203/4 tonality while adding a little extra gain and sizzle to the higher strings. Now my arithmatic is pretty bad but the way I figured it, if one wanted to bypass a 10k cathode resistor & keep the frequency responce similar to that of a 2k7/.68uf stage, then u would use a .2 or .22uf as this puts the 6db roll of point just slightly lower than that of the 2k7/.68uf stage whereas a 10k/.1uf would put it a fair bit higher. My math could be completely wrong but either way I think I'll follow your lead of trying a .2uf over the 1st 10k cathode. I am almost ready to install the board & the only components not on it are 10k bypass caps & plate bypass caps. I used some circuit board cleaner to remove the resin spatter last night & will do a few small jobs inside the chassis each evening so it right to install the board this weekend. Cheers

Thanks for the futher run down! That is why I used a 0.22uF rather than 0.1 (that was too bright and lost the lows - I did try it for myself!). It will be interesting to find out how a 50w turns out.