breadboard fun.

[DummyLoad]

odd tube fun again. lab and breadboard were getting forlorn... 

the contenders: 2 x 6MF8 - a vert. osc. & vert. amp. hi-mu triode & 6V6 on steroids- actually is 6EZ5. comparing GE tubes, has same plate structure as the 6V6GTA. triode is similar to 12at7. 

i used a PT salvaged from baldwin organ combo - it powered 3 push-pull 6bq5 amps with a 5u4gb rectifier. in this configuration it's pushing 415V to OT CT with SS recto. the 40V secondary was a bonus. i had to use V doubler for bias supply since i thought i was going to need more than 48V. OT is a tremolux 4k:4ohm replacement model running with 8ohm load for ~8k reflected.

started out with 400V to screens but bias was at -58V to hit target of 20mA per pentode. not what i wanted. to compensate, i added the screen supply divider network and now bias is at -47V for 20mA per pentode. i was working with the curves from the 6ex5 datatsheet since the 6mf8 datasheet does not present any curves. 

sounds good - better than i was expecting, but honestly i was really expecting a total flop: e.g., oscillations, instability, etc.. ex. i had to add a 100pF plate to plate coupling cap to the LTPI and cross the drivers to get rid of an oscillation tendency when pushing it or probing with a DMM/scope. IOW, V3 triode drives V4 pentode & vice-versa.

not a WOW sounding amp as is, but i think with some more tweaking it might be close. more l8r. 

http://tubedata.milbert.com/sheets/093/6/6EZ5.pdf
http://tubedata.milbert.com/sheets/084/6/6MF8.pdf

respectfully,

--pete

[DummyLoad]

BB pic. the adapters were a &!tch to make - wiring.


--pete

[kagliostro]

COOL Pete

I like your odd tube experiments 

the adapters were a &!tch to make

But now you have it and can go on experimenting with other Compactron Tube 

--

.... to compensate, i added the screen supply divider network and now bias is at -47V for 20mA per pentode

Question:
Lowering G2 voltage has the effect to permit to achieve a certain current at a higher Bias Voltage (-47v is higher than -58v) ??

Thanks

Franco

[DummyLoad]

lowering g2 supply improves sensitivity. LTPI Vp to drive to full power is reduced - in this case from -58Vp to -47Vp.

the adapters will work only with the EIA 12DZ base - only a handful of compactrons use that pinning: 6LU8, etc..   

--pete

[kagliostro]

Thanks for the explanation Pete

--

Bad news about the limited compatibility of the sockets 

I was thinking you were able to test many kind of compactrons

Franco

p.s.:

6JZ8 12DZ  Medium-Mu Triode + Beam Power Pentode
6LU8 12DZ Medium-Mu Triode + Beam Power Pentode
6MF8 12DZ Hi-Mu Triode + Beam Power Pentode
6MY8 12DZ Medium-Mu Triode + Pentode
12JZ8 12DZ Except for filament voltage, identical to 6JZ8

[DummyLoad]

well, when you wire the LTPI correctly - things just work better. reconfigured with V3 triode drive V3 output pentode and V4 triode drives V4 pentode. added NFB loop and schema attached is as built - scrap the older copy - this one works.

OT is a 40W hoffman p/n 019343 - a generic fender part with 4k with 2/4/8 secondary taps. i'm running it as 8k:4/8/16. yes, i know it's overkill, but it's the best sounding OT i have on hand. in this configuration, the MV is quite handy to have - i now see why the 2204 is so well liked.

what does it sound like? it sounds like a 2204 with 6V6 - really nice smooth tone and worth a listen.

matching: of the four 6MF8 tubes i bought all power pentodes were within 2-3mA of each other - the tubes running on the breadboard are conducting at 20mA and 21mA with -46V at g1.

checking out the LTPI in depth, i balanced it with my o-scope and i ended up with the values published that sounded best to me while still producing decent symmetry. it will drive 70Vpk very clean and flattens out at near 80Vpk.   

future plans: i just received a 18Watt PT from doug today. YAY! the vintage baldwin PT is a one shot, so i'd like build it with production iron. i plan on procuring a tremolux-like OT with multi-tap secondaries to test [too bad doug doesn't sell one! ;-) ] further in order to get the overall bulk down.i thought are that this should fit in a stout chassis since it uses just 4 tubes and the 2204 circuit wires in easier than the plexi preamp.

very pleased up to this point! happy hacking! y'all!

--pete


EDIT: resubmitted schematic: voltage indicated at V1 is an error - was 395V; is actually 295V.

[DummyLoad]

this one doesn't squeal at ~10KHz. had to go back to swapping the phase inverter outputs to the drive tubes. have just basic telemetry on this schema - i'll fill in the chart later.

now using doug's 18W PT - lost about 15V - rearranged the g2 divider network & revamped the fixed bias power supply. the bias supply has a range of about -41V to -58V: i'm going to give a pair of 6LU8 a try.

have a hammond 272DX on the way as i'd like B+ up around 415V to 425V.

cheers!
 
--pete

[DummyLoad]

making 23W right at start of flatting of the sine - 20Vpk into a resistive load that measures 8.2ohms. 

Ch 1 is set to 10V and is the load resistor - Ch 2 is set to 1V and is the signal generator. i broke off the preamp and inject signal into top of MV pot.

[DummyLoad]

overdriven.

[DummyLoad]

distortion is about 3% before the onset of clipping at about 20W. just curiuos about the performance. rolloff started at about 150Hz: was down ~1Vrrms at the load resistor. 


--pete

[DummyLoad]

schematic with telemetry - this is ver2.


--pete

[DummyLoad]

working on a layout for this one.


perfect! this is ver. 3.


probably going to go with the 17" x 6" x 2.5" chassis for this one. with this larger choke and all the iron the stout may be too packed in.


--pete

[kagliostro]

I know Pete, you like tradition

but if you want to save a bit in space and weight .......

Solid State Choke


give a try to the whole circuit inside the red rectangle (note that the 47uF capacitor is necessary, but you can use a different value)

Franco

[DummyLoad]

I know Pete, you like tradition

but if you want to save a bit in space and weight .......

Solid State Choke


give a try to the whole circuit inside the red rectangle (note that the 47uF capacitor is necessary, but you can use a different value)

Franco

K, thank you. i will give it try another time. this amp is getting the choke. i will be using a more compact one that i have already selected. i have already decided on the 17" chassis, as it's has less crowding with a cleaner layout than anything i've come up with so far with the 12" chassis.


--pete

[kagliostro]

A real choke also has some effect that a SS haven't, also if not bad results with a SS version

so, if you have the right one and enough space, that is the solution

Ciao

Franco

[DummyLoad]

revised schema attached - fixed ref des: r14 had a duplicate.


--pete

[DummyLoad]

a work in progress, missing: LTPI to power tubes; OT wiring; NFB; AC wiring; Filament string. 

layout tracks schematic attached.

--pete

[DummyLoad]

just to be sure before committing, i wired the plexi (1959) preamp back in. sure enough, i fell in love all over again.     no 2204! if in the future  I DO stray back to a 2204, y'all cyber slap me back into perspective. it's your collective duty to do so. please...      you can refer to this post.
 
--pete

[shooter]

i fell in love all over again. 

as a non-player I've been trying to figure out "sound" from schematics, specifically the 2204 vs plexi.  I have a plexi-ish and think it's the best tone box I've done. 
Could you use Player words to help me understand your "why"?

thx

[DummyLoad]

shooter: cleans with more headroom, less compression, clear and defined attack on low notes, better suited for rhythm plying. when you blend the channels and push it you get overtones not possible with the 2204 and compression. the 2204 is a great preamp; IMO, it's just not as versatile.

--pete

[shooter]

Thanks Pete, that's the language I was looking for
That's pretty close to what I hear when I play my plexi, I don't have great headroom at volume, but I'm using a tranny PI and Cathode biased 34's with lowish B+.  I can make it sound clean/chimmey as a twin, and close to middle metal full roar

[DummyLoad]

Thanks Pete, that's the language I was looking for
That's pretty close to what I hear when I play my plexi, I don't have great headroom at volume, but I'm using a tranny PI and Cathode biased 34's with lowish B+.  I can make it sound clean/chimmey as a twin, and close to middle metal full roar

another footnote: the addition of a MV as implemented in the 2204, is a worthwhile addition to the 1959 preamp.


--pete

[shooter]

is a worthwhile addition

I "lifted" my PA from a clubman, and it has a cross fade? brilliance MV, NOT worth adding

[DummyLoad]

is a worthwhile addition

I "lifted" my PA from a clubman, and it has a cross fade? brilliance MV, NOT worth adding

cross-line MV is a poor implementation of a MV.
--pete

[DummyLoad]

settling on this. i believe that the layout is mostly correct. some more eyes on it would be much appreciated. OT wiring and heater wiring not shown: i assume for now that i can't 'effthatup. 


schema and layout attached. not sure i'm going to implement elevated heaters. seems unnecessary...   


--pete

[shooter]

not sure i'm going to implement elevated heaters. seems unnecessary

I like the design, yours?  usefulness is subjective

[DummyLoad]

not sure i'm going to implement elevated heaters. seems unnecessary

I like the design, yours?  usefulness is subjective

no sir, just a simple V divider. marshall did not implement, neither did leo. however, with some of the new-production tubes i've been reading about lately that seem to have issues with Vh-k, it seemed like a good idea. there are plenty of NOS 6MF8 floating around that are cheap: 12ax7 not so much.


--pete

[kagliostro]

..... 12ax7 not so much

Fortunately there are also 6n2p

https://tubes-store.com/advanced_search_result.php?keywords=6n2p

Franco

[DummyLoad]

thanks franco. 


--pete