Question on KT-66 vs 6L6 vs EL34K

[markmalin]

Hey guys,


I'm working on an amp which I use KT-66's or 6L6's.  The transformers I'm using are:


PT - Hammond 290DX, 325-0-325, 50, 5, 6.3
OT - Hammond 1750Q, 7,371 Ohm primary (425V) 50W


My question is, I kind of designed this for the KT-66's but I'm not sure I'm liking the tone.  I realize KT-66/6L6/EL34 are all sort of drop in replacements, but is there anything I need to be aware of before trying EL34's in this amp, for example?  I guess what I'm wondering is, will the plate voltage be ok and the reflected impedance be "happy" with any of these tubes.


I don't have a plate voltage measurement handy - I can measure that next time I have the chassis out.


My apologies for the rather vague question - just swamped building other stuff, not happy with the sound of this amp, and figured I'd try some other power tubes (and sitting here at work on a Friday morning wishing I was in my shop :(


Humbly,
Mark.

[Ed_Chambley]

Most guys use a 4 to 5K OT primary for 6L6's.  EL34's 3.4K.  This is if you prefer max power.  If I were to sub EL34's I would change the secondary by doubling it.  If your OT is for instance:
7.4k/8 ohms then is would be 3.7K/16 ohms and this would be right up EL34's alley, provided your loaded voltages are 425+.

6L6's would work well with this arrangement as well.

[sluckey]

The EL34s will need a change of bias and will require pin 1 connected to pin 8 (or ground or some negative voltage).

The EL34s would probably be happier with an OT that had a 3000-5000Ω primary impedance (depending on plate voltage), but that can be manipulated with secondary loading.

[markmalin]

Thanks, guys.  I appreciate it.
Fortunately the OT is a multi-tap and I'm running 4 Ohm speakers...so  this will work well for a trial.  I'll just set the impedance switch to 8 Ohms and re-bias it -- and make sure pin 1 is connected properly for the EL34's.  This is helpful...now I can do some experimenting with different tubes to see if I can get a more pleasing sound :)
 
Mark.

[lkrasner]

What is it you don't like about the sound?

If you aren't after huge power, and the bias setup will allow, some smaller tubes like a 6V6 may be a good choice. You'll get less headroom, but I generally like the breakup better than the bigger tubes. A lot of people disagree with that though.

[markmalin]

What is it you don't like about the sound?

If you aren't after huge power, and the bias setup will allow, some smaller tubes like a 6V6 may be a good choice. You'll get less headroom, but I generally like the breakup better than the bigger tubes. A lot of people disagree with that though.

It's a long story... it's kind of a strange amp in that it's based on an AB763 style BF Vibrolus with a Lar-mar master volume and a Dumble overdrive circuit, then it's going through 2 10's.  I'm  looking for a "throaty" blues tone, hence the KT-66's. 

- Basically I can turn the Master down and the normal volume up and get clean tone - that's good.
- I can turn the normal volume up and the Master down and get power tube style overdrive, which is a function of the Lar-Mar style master.  It's not a vary attractive overdrive as you might expect - Fenders dont' sound good this way
- If I set for some clean headroom (normal vol down, master up) and switch in the Dumble overdrive, it's "ok" but more muddy that I'd like.
- The amp has too much bass, which I tamed somewhat by lowering the bypass caps on V1 to 5uf rather than 25uf (Fender style), but it's still not a pleasant tone...and turning the Bass from like 1 - 3 is all you'd ever need.  Beyond that it's not a bad sound, it's just too much.  I need to address that (expect more posts! lol)
- The other weird thing is, the reverb is just too "lively".  It gets way "surf" if you turn it up past 5, but it's too bouncy...like if you have a digital reverb pedal and turn the "liveliness" knob up way too high.  Again...more posts to come.

I just thought for curiosity's sake I'd like to remove the KT-66's from the layout.  I built one of these before (and sold it) and it really sounded nice and bluesy, it's just this one isn't doing it yet.  Again, it's "acceptable", but not what I'm after.  It's almost like it's too harsh.  It's not a warm tone, sort of shrill in the middle frequencies.  And the OD is too flabby sounding.  I want more Strat/texas blues kind of thing.   The other deal is, I removed the 2 Celestion Greenbacks and stuck some Jensen Jet's in there, and I think I need to ditch those asap.  That may be the "shrillness" I'm experiencing.

So...you asked!  Sorry to ramble, it's just bugging the heck out of me and I need to get this circuit right.  I've got a customer order for this model that I'm working on for an artist endorsement.  He's specifically asked "can you make it less bassy" and "the reverb is too intense...".
 
Mark.

[Willabe]

- The other weird thing is, the reverb is just too "lively".  It gets way "surf" if you turn it up past 5, but it's too bouncy...like if you have a digital reverb pedal and turn the "liveliness" knob up way too high.

Any Fender AB763 I've owned/played through once you get past 3 1/2 on the reverb it's really strong sounding.

I built one of these before (and sold it) and it really sounded nice and bluesy, it's just this one isn't doing it yet.

What did you change between the 2 builds? Iron? Speakers? Circuit(s)? Parts types/brands, resistors/caps?


                   Brad     

[markmalin]

- The other weird thing is, the reverb is just too "lively".  It gets way "surf" if you turn it up past 5, but it's too bouncy...like if you have a digital reverb pedal and turn the "liveliness" knob up way too high.

Any Fender AB763 I've owned/played through once you get past 3 1/2 on the reverb it's really strong sounding.

It's not a nice reverb sound.  Over lunch today I stopped at GC to play a new Deluxe Reverb and the reverb is very intense past 3, but it still sounds deep and good.  Like in a deep room. The amp I'm working on doesn't sound so much like a room (or big hall) when you turn the rev. up, it's more like lots of intense short echos.  Hard to describe, but I my post a recording if I have a chance.

I built one of these before (and sold it) and it really sounded nice and bluesy, it's just this one isn't doing it yet.

What did you change between the 2 builds? Iron? Speakers? Circuit(s)? Parts types/brands, resistors/caps?
                   Brad     

That's the crazy thing. I had records from building the first one, so I wanted to reproduce it.  Same parts, though the coupling caps may be PS's instead of 716's (Sprague Orange Drop).  I know polyester vs polypropoline - one can sound harsher than the  other. 


Differences
- Accutronics doesn't build the same reverb tanks they used to from what I can tell. Minor, but it's different.
- I'm using the Lar-Mar style Post phase inverter master volume.  That said, I'm fairly certain the final version was like this on my last amp.  That's one thing I couldn't find proper records for in my documentation. (live and learn). 
- I may have a smaller negative feedback resistor (I was at 10K on the last amp but it was too much punch, so I added a "lame" switch to go between 10K and 1K, I believe). 
- On this version I've lowered the V1 cathode cap value as I mentioned before to get rid of some of the flabbyness in the bass.


Interestingly enough, when I finished this one there was a parasitic oscillation that when you turned up the volume too far it would feed back.  I shortened the lead from the Negative feedback resistor to OT and that stopped it.  Heck...maybe it's still there but I can't hear it and it's killing the tone?


Mark.

[shooter]

When you swap around PA tubes and dial in the bias, you also need to check/change the drive to them.  I did sorta the same thing you're doing, 6550 to EL34 to KT88, back n forth.  My "sound/tone" finally showed up when I added a small gain stage that wasn't there, now I'm back to the original 6550.  fwiw