1700 ct OT and 6l6

[Chiamp]

hi guys , new here ,

I have a project that I built where I use a marshall style 1700ct pri 4-8 16 sec OT that I got for good price and PT is an old BF twin 450v on the plates
and using it with 4-6l6 and it has around 300v on PI plates and 195v on the 100K and 150K preamp plates with matching cathodes and 22Uf cath caps

the phase inv is classic blackface input cap int PI is .0047 all three .1 caps 470r 22k
the nfb resistor is now 10k from 8ohm tap and tail resistor is 400r I tried some different nfb values no difference ,but this is what in now and sound sthe best so far

and also has clean preamp section that is basically a normal channel non reverb fender and EQ but with all mesa MK2 values 25k mid pot and liftstack boost ala old mesa caps are 330p .1 ,047

my main problem is that I get a buzzy crossover distortion when played loud and gets worse when boost engaged and if a Tubescreamer type overdrive pedal is kicked in
I tried to bias it out but even at higher setting it remains now set at 34ma per tube i lowered the 220K bias resistors to 150k it got better but still there lowered the Pi input cap from .01 to .0047 it got better
I got 4k7 bias gridresistors currently it starts happening when master is about one aclock and gets worse the more you crank it up but below its sweet as an old BF Twin

i thought the 1700ct trannie might be wrong for the 6l6 , could it be that simple that it 6l6 grids loads too soon ?
would that be a more narrow swing basically ?

and would it be possible to correct the impedance reflection like changing somehow maybe lab more with NFB or higher input gridresistors etc ?
all to better suit the 6l6 that I really want to use rather than el34s

it sounds really sweet until it starts to distort then its buzzy and harsh,

any ideas appreciated

Chiamp

[ziggy007]

hi mate,
I'm still very much on the vertical learning curve myself but had the same set up on a modded bandmaster i fixed last month.(4 6l6 & 1700 ot) and it worked fine on the 8 ohm tap. all my components are still original spec. It was running 460 on the plates and right on spec bias wise (-52v: about 70% )
not that this helps you much but ya never know
Cheers
Gavin

[Chiamp]

Thanks Gavin,
what speaker load do you run on your 8 ohm tap, 8ohm ?
and did you use 820R nfb ?
did you find it necesary to bias full 70% to avoid Crossover Dist
or was it just for best tone

I am really curious how much I really must consider to use a 1700 OT compared to a 2K when using 6l6 to be closer to the best results considering the "wrong" OT ,
could screen resistors resistance (470r) or choke rsistance change anything in the curves how it responds to input signal on the grids or even change in cathode resistor 1ohm to maybe 10 or even 100ohm ??

is it so that 1770 and 6l6 makes it so much more sensitive because of the different load ?

[Merlinb]

Your description sounds more like blocking distortion than crossover, to me. Try some bigger grid stoppers on the 6L6s. 10k to 47k maybe?

[ziggy007]

Yeah just a 8ohm on the 8ohm tap. The owner was thinking about getting me to mod it back to the original bandmaster specs (2 x 6L6)but changed his mind.
I am still very much a novice in (repairing) tube amps so asked for help earlier on (search for my earlier '63 bandmaster threads) the guys here were very helpful and I learned heaps.
Some of the earlier Twins had 4 x 6L6s and had a similar OP Trannie impedance.
The 70% just happened to be right on spec and the owner wanted it there (& it sounded sweet). The NFb was 820.

[Chiamp]

Your description sounds more like blocking distortion than crossover, to me. Try some bigger grid stoppers on the 6L6s. 10k to 47k maybe?

Thanks,
well there is a crossover notch on the scope I lent ( not very familiar with using a scope but it has the typical crossover kinks when I crank it up it basically "buzzes and vibrates" on the sinus wave too when full up,
well I will try some higher gridstoppers,

I am really would like to know the functions on these tubes operate on this load, any ideas without getting to complicated ?

[Chiamp]

Yeah just a 8ohm on the 8ohm tap. The owner was thinking about getting me to mod it back to the original bandmaster specs (2 x 6L6)but changed his mind.
I am still very much a novice in (repairing) tube amps so asked for help earlier on (search for my earlier '63 bandmaster threads) the guys here were very helpful and I learned heaps.
Some of the earlier Twins had 4 x 6L6s and had a similar OP Trannie impedance.
The 70% just happened to be right on spec and the owner wanted it there (& it sounded sweet). The NFb was 820.

Thanks ,
interesting, I did not know about the lower pri-impedance on early twins,
do you know the actual specs on those ?

[Merlinb]

Thanks,
well there is a crossover notch on the scope I lent ( not very familiar with using a scope but it has the typical crossover kinks when I crank it up it basically "buzzes and vibrates" on the sinus wave too when full up,
well I will try some higher gridstoppers,

I am really would like to know the functions on these tubes operate on this load, any ideas without getting to complicated ?

A novice with an O'scope, now there's a thing!

Crossover distortion of itself is not such a bad thing- it's all part of the PA distortion pallate. It's only when its gets REALLY bad that it becomes unpleasant. A modest kink is fine, but a sort of cathedral-spire ziz-zag is going too far. Does yours look really bad?

Your description of "vibrates" sounds a little worrying though... What does a square wave look like?

[Chiamp]

A novice with an O'scope, now there's a thing!

Crossover distortion of itself is not such a bad thing- it's all part of the PA distortion pallate. It's only when its gets REALLY bad that it becomes unpleasant. A modest kink is fine, but a sort of cathedral-spire ziz-zag is going too far. Does yours look really bad?

Your description of "vibrates" sounds a little worrying though... What does a square wave look like?

ha ha , well just got me an old analog scope recently and trying learning how to use it properly and learning the different signs etc , but its very useful indeed, and fun to pick up real world info from you guys would be cool to see if anyone has a pic of a good cranked fender style amp on a scope how it should look

anyway I will check with square wave , right now it does looks rather bad when amp is on full tilt ,
up till about 12 its looks and sounds ok closer to 3 aclock it shows the typical lowbias amp notches and a slight notch and tilt on the wave
and when full the signal starts to vibrate and sinus waves goes skyhigh ,

[Chiamp]

A novice with an O'scope, now there's a thing!

Crossover distortion of itself is not such a bad thing- it's all part of the PA distortion pallate. It's only when its gets REALLY bad that it becomes unpleasant. A modest kink is fine, but a sort of cathedral-spire ziz-zag is going too far. Does yours look really bad?

Your description of "vibrates" sounds a little worrying though... What does a square wave look like?

checked the square wave now, I had the master vol on full gradually raised the pre vol
and on low vol it has a pretty sharp almost vibrating mid dip notch
(Eq is quite scooped btw ) but the mid notches looks a little distorted from what I can tell and when i raise the vol the mid nothes goes away it gets closer to squared when a bout 12 on pre vol and when full it is squared but slightly vibrating when dimed ,
did the same but pre vol on full and gradually raised the master same result , btw the OT sqeels (quietly) but still pretty bad when full signal is applied

I have also check for leaky signal caps, but all ok

[Chiamp]

upped the gridstopper to 15k, not much difference

search goes on ,

anyone who might have input on my original question about four 6l6 into 1700 pri impedance if this is more sensitive to higher signal than 2K

also does screen current affect this ?

[tubeswell]

There isn't much difference between 1k7 and 2k primary Zed. Both are in the same ballpark.

Try a higher grid stopper (47k - like Merlin said earlier)

[Merlinb]

upped the gridstopper to 15k, not much difference

search goes on ,

anyone who might have input on my original question about four 6l6 into 1700 pri impedance if this is more sensitive to higher signal than 2K

also does screen current affect this ?

The lower impedance OT will put you a bit closer to class B and crossover, but not a lot.
The 'vibrating' signal you are seeing seems indicative either of ringing (NFB loop or not enough filtering in the PSU) or bad ground or cold solder joint somewhere. OR it's just some rectified hum riding on the signal and nothing to worry about. Hard to say without seeing it.

How does it sounds with the NFB removed?

[Chiamp]

The lower impedance OT will put you a bit closer to class B and crossover, but not a lot.
The 'vibrating' signal you are seeing seems indicative either of ringing (NFB loop or not enough filtering in the PSU) or bad ground or cold solder joint somewhere. OR it's just some rectified hum riding on the signal and nothing to worry about. Hard to say without seeing it.

How does it sounds with the NFB removed?

hmm closer to Class B
Great, this is exactly the info I wanted and it really helped me to better understand some of what going on ,

ok, I today upped the main psu caps from 50uf total to 100 and played with the phase inverter voltages and cathode resistor and also tightened one of the cathode chassie solder lugs for one of the powertubes ( was not 100% tight !) I tried disconnect the NFB at first but no difference, only worse and louder, the amp is rather loud btw

but now it works a lot better , the vibrating is gone and amp responds faster and smoother, still when the amp is dimed the sinus wave get fully squared, not just on the wave tops but it really turns into a square box LOL
so it sounds a little over chrunchy, but I suppose I will not dime it but I will try solve that some how, ideas welcome
It is also biased rather hot now about 45ma pertube,


I need to learn more on how to read the scope and more info on how power tubes operate with different impedances etc

pix of good scope waves would be nice to see if anyone has for example a dimed fender style amp and marshall style amps just to see
how it should look for a compresed nice cranked tone, I guess it subjective but ideas are appreciated

Thanks a bunch guys,
you help have learned me a lot and steered me into right direction and also gotten me search for more info on the web and on this site !!!!

very cool

[Jazz P Bass]

The old Fender amps hit the top of the power rail pretty early, like 7 on the volume.(110 mv input). Vintage! Sounds bad to my ears.
This is seen on the scope as a flat top & bottom.
Is this what you are seeing when you "dime the amp"?
Then again, why are you trying to "dime it"?