LITE 2 PARALLEL/DRIVE

[geoff 1965]

update to tweaking the drive/cascade stage,i mentioned earlier i was'nt 100% happy with the 22nf coupler and 1K5/25uf cathodes.i was getting some nice overdrive on the bridge pickups of both guitars but when switching to the neck pickup the clipping became "mushy"
so i've just tried ZP's method of hot and cold biasing with a massive improvement! now the overdrive is clearer and more defined on both bridge and neck pickups. on the 1st triode i used 2K7 with a 1uf mallory film and on the 2nd triode used 820R with a 10uf electrolytic.these were simply the values i had to hand so i'll order 680R,0.68uf and 3.3uf and do a bit more experimenting.i'm not in a rush to finish tweaking because i'll have to sort that "bloody" heater wiring out next!!
note; with the 1K5/25uf on each cathode the parallel was virtually the same as 820R/50uf shared with just a slight cut in bass but still sounded great!so when i get the components to try ZP's hot/cold biasing i'm hoping to keep the parallel as good because that's more important to me! this has been a very good learning process for the effects of coupling caps and biasing when "tone shaping" an amp and with a bit more experimenting i'll find the right combination for both parallel and drive.

[kanj]

Geoff 1965

How's the progress going on the "tone shaping", and in particular, it's affect on the "parallel" triode operation?

Are the mods using ZP's component values altering the tonality of the the amp when in parallel mode (as opposed to the "cascade" mode)?

I'm really interested in your thoughts and the benefit of your experience/experimentation as I still need to obtain a few parts (like 3.3 uF caps !) and a few other odds and ends - and would certainly take your results and recommendations!

Thanks!

kanj

[geoff 1965]

hello Kanj,
i'm just waiting for the 680R resistor and 3.3uf's to land before i can compare,i've ordered both electrolytic and film 3.3 caps to try also some 22uf and 4.7uf's in case i'm not happy with the parallel with ZP's values.that's my main concern! the parallel with equal biased cathodes was nigh on perfect and very good in drive but a bit "mushy" on the neck pickups so if the hot/cold bias alters my parallel too much i'll go back to equal bias and try a pair of 22uf's or less maybe?
from the start i've gone for the "warm,driven marshall tone" with the amp,i'm not bothered with "headroom" or "clean vox chime" that's just my preference! i fitted the sag resistor which has a knock on effect with the B+! my el84 plate voltage is 344 with good dissipation of 10.80 watts but i've dropped about 10 volts on the preamp so that in turn will will alter the biasing slightly.i will replace the 8K2 resistor with a 3K3 and that will bring the preamp plate voltage back to around 140-5 in parallel,i'll post the tweaking results regarding the cathodes as soon as i can.
looking forward to seeing your build start!

[kanj]

Geoff 1965,

I really appreciate your thoroughness and patience in your tweeking endeavors! You have to be learning a lot about how and what makes your amp sound a certain way with all the component swapping and testing (I hope I have as much patience!).

If I remember correctly your Cage 2b has a solid state rectifier... did installing the sag resistor increase the touch sensitivity any? Or does it allow for more compression and a singing quality as the power amp gets pushed? Your thoughts?

As an aside, how were you able to figure out swapping out the 8K2 resistor for a 3K3 will bring the V1 parallel plate voltages back up to 140-145? Is there a formula for figuring out such things regarding the power supply?

I was reading in one of Gerald Weber's books that lowering a tube's voltage "warms" up the sound, whereas running them at a higher plate voltage gives them more headroom and cleaner sound at the expense of making the sound more sterile or brittle (not sure I am describing that right, but hopefully you get the idea). I am sure there are a range of acceptable voltages (within reason) depending on what one is trying to accomplish. You mentioned that your preamp voltages were down 10 volts... did you notice more "warmth" with the lower voltage? Also wondering if that would contribute to the mushiness you noted when playing in the neck position?

Don't mean to hit you with a bunch of questions... just trying to make sense of what I'm reading vs. reality!

Thanks for posting all your changes and what you are doing! (and answering my endless questions!).

... and, Yes! I am looking forward to getting going with my build... I should be able to get a good start at the boards tomorrow, maybe even punching out the chassis before I have to travel for a few days this week!

kanj

[geoff 1965]

hello Kanj,
the sag did make a difference to the touch sensitivity just like Curtiss said it would,and i'm not sure about compression when the el84's are pushed so i'll try that area when i've finished tweaking the cathodes.all the testing i have been doing i've kept the power tubes fairly clean and used the celestian 100 watt so it does'nt break up.that way when i'm experimenting with the gain stage all the overdrive is coming from the preamp which makes it easier to hear the cuts in frequency etc,we all know how the el84's sound when driven!
the mushiness with the neck pickups was there before i fitted the sag resistor and it's just finding the right balance between the cathodes and the coupling caps.
there's so much controversy with tube voltages,"uncle doug" on you tube says bringing the dissipation to around 70% gives you better headroom and prolongs tube life,but he's obviously a fender man!
i think it's another area that's down to personal preference i.e. mine are 10.80 watts which is 90% but they sound good to me and actually before i tweaked them they were borderline redplating at nearly 100% dissipation but still sounded good! not everyones "sweet spot" is the same and the beauty of building your own amp is being able to "voice" it to what you like.
now onto the B+ adjustment! there will be a formula using ohms law or something but i don't know how to do it! just get a couple of variable wirewound resistors and make a little tester,the one i made has 0-1K which is perfect for el84 biasing and 0-1meg which is perfect for the B+, i took out the 8K2 and patched in the 0-1meg and turned it until i had 320V B+ then measured the resistance.

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hope this helps Kanj and when i finish with the cathodes i'll do a more in depth test with pushing the power amp & sag resistor.

[kanj]

Geoff 1965

I think your approach to shaping the preamp gain makes perfect sense... ie being able to isolate what's coming out of the preamp (and tuning that part of the amp)... and then the power amp section.

Relative to biasing the power tubes - from what I've read - 70% is the "standard" recommended for pp output tubes - however it is a matter of choice relative to what sounds good (and what one is willing to live with relative to tube life/longevity). Before I had any idea about "proper" biasing I used to push the biasing of the 6v6s in my Princeton to give it a little more headroom and sparkle because it sounded better to me (fortunately I didn't fry anything!).

Thanks for sharing your test device! I recall seeing that earlier in this thread .... That's brilliant! I'll be adding a couple of variable wirewound resistors to my next parts order to make one!

kanj

[rock_mumbles]

Output tube biasing depends on how the tube is biased ...
If the amp is fixed bias, bias the tubes between around 55% to 70%
If the amp is cathode biased, bias the tubes up to 100% dissipation ...

BTW, larger screen resistors give more compression, clean up the overdrive, and also can prevent red plating

[geoff 1965]

thank's Rock,
that's good to know,150R gave me 100% and 180R i have now gives 90% and 344 on the plates which i'm happy with. i'll have to remember using larger screen resistors for more compression etc.i would have thought you would have to exceed the max 12W dissipation to be in danger of redplating or is there any exceptions?

[geoff 1965]

Ampaholic's!
i dropped 10 volts on the preamp when i fiited the sag resistor so have just swapped the 8K2 for a 3K3 and now i have my preamp B+ back at 320V.
i've been testing different biasing on the preamp tube and thought it would be of interest to post the results,i did'nt expect the biasing to effect the plate voltages as much as it does!

1st triode-2K7/.68uf parallel: pin1=129 drive: pin1=228 pin6=157
2nd triode-680R/3.3uf

1st triode-2K7/.68uf parallel: pin1=159 drive: pin1=229 pin6=197
2nd triode-1K5/22uf

1st triode-1K5/.68uf parallel: pin1=140 drive: pin1=198 pin6=197
2nd triode-1K5/3.3uf

1st triode-2K2/.68uf parallel: pin1=133 drive: pin1=219 pin6=166
2nd triode-820R/3.3uf

1st triode-2K2/.68uf parallel: pin1=153 drive: pin1=220 pin6=198
2nd triode-1K5/3.3uf

[kanj]

Geoff 1965 wrote:
i've been testing different biasing on the preamp tube and thought it would be of interest to post the results,i did'nt expect the biasing to effect the plate voltages as much as it does!

Interesting! Did you by chance listen to the amp with the different biasing on the preamp? Just wondering what effect it had on how the amp sounded with the different voltages & components?

rock_mumbles wrote:
Output tube biasing depends on how the tube is biased ...
If the amp is fixed bias, bias the tubes between around 55% to 70%
If the amp is cathode biased, bias the tubes up to 100% dissipation ...

BTW, larger screen resistors give more compression, clean up the overdrive, and also can prevent red plating

Thanks for clarifying what's allowable in output tube biasing of fixed vs. cathode .... and for the info on the screen resistors and their effect!

kanj

[geoff 1965]

Kanj,
yes i've been listening to the effects of different biasing,the lower value caps are better for bass response and cured the"mushy"overdrive, with the 25uf's i could'nt get rid of it with the drive coupling cap without altering the overall sound.that's the best way i can describe it so i hope that makes sense!i used a Sozo 0.68uf on the 1st triode and a polyester film 3.3uf on the 2nd triode,these are a good match for the higher value 10nf and 22nf coupling caps.
notice how much the parallel plate voltage is affected by the biasing.i'm leaning towards equal biasing for the best parallel but i also like the hot/cold! i tried 2K2 and 820R and was impressed with both parallel and drive, don't forget my plate resistors are both 120K and i don't have the balance components in ZP's mod so take that into account.
i quickly went through the various biasing values to get the voltages and a rough idea of sound,but i have just spent a couple of hours with the cathodes equal at 1K5/.68uf and 1K5/3.3uf and could not find fault! the bass response is very good in both parallel and drive,parallel sounds great with the el84's driven or cleaned up with the PPIMV,cascaded overdrive sounds great with both neck and bridge pickups "no mushiness" so that just leaves the hot/cold biasing to try before settling but i think this is the one for me! i did'nt like the parallel with 680R and 2K7 but 820R and 2K2 was good so i'll spend a couple of hours comparing to the equal biasing.

[kanj]

Geoff 1965,

Love this thread.....It's awesome that you are going through all these comparisons, measuring your voltages and seeing what & how various parameters affect the preamp in each mode.

Thanks for posting your work and observations! (ALL this stuff is going in my notes!)

kanj

[geoff 1965]

spent a couple of hours experimenting with the hot/cold biasing today and they are pretty good i.e. 680R/2K7,2K2/820 etc but this again will be down to personal preference!i've gone for 1K5 equal biasing purely on the parallel being awesome!
also used shielded wire to and from the gain pot just as a precaution,have the gain resistor to "fine tune" tomorrow and that's it lad's!

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[kanj]

Geoff 1965 wrote: i have just spent a couple of hours with the cathodes equal at 1K5/.68uf and 1K5/3.3uf and could not find fault!

So if I am understanding your most recent post correctly, after trying the hot and cold biasing you have returned to the values listed above for the biasing on the preamp?

I apologize for the question... I've got some stray parts to get ordered for my build and just want to make sure I understand what I need to get as the parallel "mode" is probably what I'll end up using the most (at least that is my thinking at this time), with the cascade "mode" as a nice extra feature to have. Also, I have interest in being able to have my amp be able to do "warm and clean", which I'm hoping the 1k5 allows for?

Any recommendations for other parts values that might be useful relative to the preamp.....?

Thanks!
kanj

[geoff 1965]

Kanj,
my original intention was the same,i alway's played through the parallel and never used the single input and saw this as a bonus but you will be suprised at how good it is!
regarding the biasing,get ZP's values 680R/0.68uf and 2K7/3.3uf the cap values are a perfect balance for the 10nf and 22nf couplers and if you get some 1K5,2K2,820R resistors you can try equal or hot/cold biasing yourself and see which you prefer.i could have gone on for days trying different configurations but the 1K5's gave me what i was looking for.
i went through all the original posts about the parallel/cascade mod and there was quite a few different options i.e. ZP originally had 680R/3.3uf on both cathodes and said this was biased warm for a thicker crunch tone boosted!,another guy used 2K2/1uf and 680R/3.3uf so you could try these.
i would use a film 3.3uf,i tried an 3.3uf/100volt electrolytic which was barely the size of a resistor and did'nt sound good!
also wire across to the tube and dpst switch from the plate resistors and don't let the wires lay against the chassis and you know to use shielded wire for the parallel signal and also to and from the gain pot.

[kanj]

Geoff 1965,

Thanks for the recommendations, posting all your experimenting, and all the advice for this ....and all your other replies & answers to my numerous newbie questions!

As you have written before, being new to this and trying to really understand what's going on has certainly "popped" some brain cells along the way.... hopefully I'm starting to understand a little more also.

Thanks very much,
kanj

[geoff 1965]

Kanj,
yeah i'm going to rest my remaining brain cell's now and just play my guitars!
best of luck with your build

[geoff 1965]

Pilgrim's
finished the last tweak to the drive stage resistor! this will depend on the pickup output of your guitar,my strat has 6K ohms and the guitar with humbucker's has 9K neck and 16K bridge so i've used 400K as a nice balance between the two.
here's the updated schematic and layout.

PREAMP LAYOUT.jpg

PREAMP.jpg

pin layout.jpg

thank's to ZP and the guys who worked on the parallell/cascade mod,that was a useful reference.
Big thank's to Curtiss and Rockmumbles for their help and good advice.

[geoff 1965]

Hello pilgrim's
just posting an update for the preamp cathode biasing,i settled at 1K5/.68uf on the 1st triode and 1K5/3.3uf on the 2nd which was good for both parallel and drive and cured the "mushiness" i was getting with the drive signal but i was'nt 100% happy with the parallel,it was a bit "thinner" sound compared to 25uf on each cathode. so after hours of testing again i've found a better solution with 2K2/1uf on the 1st triode and 1K5/10uf on the second and i have a nice full tone on the parallel and the same on the drive signal with good bass response with no mushiness.plate voltage is 152 parallel so the 2K2 raises it by around 12 volts.

[geoff 1965]

Hello,
i've finished the final tweaking to the cathodes and gain stage on the cascade/parallel mod! i added a dpst switch for the cathodes so i could switch between film and electrolytic caps,the 1uf/3.3uf film caps are superb for response and clarity especially on the gain stage but on parallel you lose the warmth you get with the higher value electrolytics.
i initially tried the standard 50uf/820 on the second half of the switch but the shared cathode was no good cascaded,i was getting a kind of "motorboating" effect so eventually settled at 10uf electrolytic on the 2nd triode which has turned out to be good in both parallel and cascade.with using the 10nf to volume pot cap instead of the standard 4n7 anything above 20uf on the cathode creates "mushiness" when cascaded.
just a quick note regarding the gain stage,the series resistor in the gain stage was quite a challenge,i was getting the best tone/clipping from the lower value 220 & 280K values but the signal was too high so i used a 400K to reduce it which worked but altered the tone.so thank's to you guy's and learning about voltage dividers i went back to 220K for the best tone and added a 220K divider which has worked perfectly! got some very good tones and level of drive now.

GAIN STAGE.jpg

NEW CATHODE BIASING.jpg

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