Discussion on a build, stereo, Parallel SE, Stereo Cab for Guitar

[Ed_Chambley]

I need to make an amp for a guy who loves SE amps, but this one has a twist.  He wants a SE type plexi circuit preamp married to parallel EL34's each with their own transformer, but running a single speaker.  This is the right side of a closed stereo cab I made for him with a V front.  Actually it looks like an old early 70's Pontiac front.  He wants to be able to switch (relay) this for a half power switch.  Not sure yet how to keep the impedance correct since removing one 4 ohm tap will drop the resistance to 4 ohms for an 8 ohm speaker.  I am planning to order Edcor ot's, so I may be able to 2, 3.4K OT's with each having a 2 ohm and a 4 ohm tap.  This side will have a tube driven overdrive.


One input in the center and consider this the left side.  I play one of these.  Basically a Brownface Preamp with 2 5881 parallel.  I checked out 2 transformers for this and it wasn't necessary.  I simply used a true 15 watt SE gapped OT with an 8 ohm tap and when the parallel 5881 switches off you get firmer bass.  This one does have a effects loop, at least mine does and he wants it.


I want to use a single power transformer and that is 275-0-275, 6.3 4 amp heaters.  This amp will have 3 12Ax7, one EF86 and 2, 6SL7 along with 4 power tubes (2 EL34 and 2 5881).  First question.  Would it be best to use 6L6GC to get a the power more even.  Never built a Parallel SE EL34 so I am not sure what wattage I will get at the end.  Sure I can read a data sheet.


I believe the PT will work fine, but I will need to have more heater current.  The PT has 380ma for HT winding, so I feel safe using it.  I did put it under load on my breadboard and tis is what I got measuring current.  I had a total of 322ma full throttle and the transformer was cool and quiet.  I may simply set up the preamps with DC filaments, but I measured 8.42 amps for the heaters.


I ended up with 374 VDC on the plates at about a total 6k (little less) load using HEXFRED diodes.  He also wants to be able to have a balance adjustment to mix the signal to a level output, but I don't know about this and what I will have to do to make this sound nice.  I consider if the impedance is equal it will split, but I cannot help but think send half of the signal to either amp will do the final tone any justice.  Fill me in.

So, I know DL loves SE and a few others have made some really cool ones as well.  Dave and Shooter and many others.  If you tubers would just chime and post some of you favorite SE schematics, even if your power tubes are larger or smaller.  I am needing some input on proven SE amps that will play clean, but will have a bit of second and slight 3rd harmonics coming from the El34. 


Thinking with 2 EL34 Mullards it will work fine, my concern is I have not tried my EF86 overdrive in an SE amp.  As a matter of fact, the biggest SE amp I ever play is a Vibro or Tweed Champ., but you cannot play with a drummer, or at-least the Drummer in his band.  Plexiglass surround.  So I do not know if this tube will work well with 1 and 2 EL34's.  When I was working it out on lower power the EF86 had a very rough tone, but did not with Parallel EL34's.


I want to use a chassis 19 x 8 x 3.  I built a stereo SE HIFI in this size chassis putting the PT rear center and OT at the outer corners which is common in HiFi, but I did not use different types of output tubes.


This sort of explains the idea and this all started with a a simple 22 degree V Front, 2 10 Cabinet ported and 2 warehouse 10 they make like Greenbacks.  Sound very convincing in the lower mids and bass.


So hit me in the head if this will not work, but since I have built half of it, it should.  BTW, the cab is 22 tall and 20 wide, pine and Burch floating Baffle and I ported bottom left and right.  The cab sounds great with a 5G9 paired with a 6G3 so off to the deep end with excessive part costs per watt.


So along with ideas, if anyone has worked with SE amps enough and knows how to keep a solid under 82hz bass tone and the type and size of the needed OT, this would be nice.  I can order Edcor, but maybe someone has build a better SE with iron designed for HiFi.  I was just going to use 4 OT's I have.  2 are 3.4k to 8 ohm for the Brit Tubes, really a 5k sounded better. and 2 6K - 8 ohms, but if you cut the OT and do not change the reflected impedance the amp will get loose and makes a E major chord sound lazy.  Suggestion from experience with these I would really appreciate.  Even if someone think higher voltage is needed to gain headroom in the preamp, let me know.


There will be no master volumes, I can assure you of that.  I have attached a parallel EL84 amp that makes a great practice amp or studio amp as it is quiet but breaks up too easily to gig with.


Also, please lay it on the line.  I assure you my panties will not wad up.

[PRR]

> make an amp for a guy who loves SE amps, ...parallel EL34's each with their own transformer....my panties will not wad up.

Check if the customer will have a panty-wad. Assemble ALL that iron (or same-weight), caps, bag of pots, duct-taped to a big chassis in a cabinet, and see if he *really* wants to carry it around.

One SE 6550 with minimal frills was enough to make me reconsider P-P.

I would not worry about impedance. He isn't going to put a meter on it and complain it is not exactly 50% power. And 50% power is not that much softer. A 2:1 mismatch will be maybe 35% power which is more like a useful step-down.

[shooter]

fwiw
I built a stereo KT88 using a 5E3 pre, was my early days so I didn't know much
B+ was too low ~340vdc plate, broke up early, good for the guitar guys that test drove it, but @50lb and more money than the 4 guys could come up with in a month it got re-purposed

next big SE, Qse EL84's, again my B+ was to low 260ish, no pre, just a 6sn7 driver.  very good audio til breakup, then it sucked, pretty high freq roll-off @~15khz.

what I've figured out, bigger B+ is better, as you know.  each tube gets it's own Rk, and tweaked using || R's, or mouser 1.257k kinda R's.  You want to match bias as close as possible.  neither of those build got me more than 20W clean audio.  For guitar (meaning fully dimed and well distorted) ~28W

I believe you'll find with great speakers and a solid build it will "sound" nearly as loud as most PP at twice the rating, so 28W SE = ~50W PP

I've only used Edcor or Hammond new iron for OT and have never looked back, they run cool even at their rated max.
build it on a roll-around amp stand, 50-70lbs gets ugly after a few days of lift, flip, move......
good luck

[shooter]

I know the customer is always right...but
if you got a BB, put together any PA tube pSE. drive them with anything you want.  now reconfig to PP self biased, repeat exact input.  tinkerin, I've found the "SE" sound comes out nearly as well as pSE, til 9, 10 there is a difference, anyway FWIW

[Ed_Chambley]

First, let me address the weight.  My friend has a very nice house gig and leaves his amp.  The only carrying he does is to his car and to my house if it needs a tuneup.  They have 2 techs that do the heavy lifting.  Currently he plays through a UL KT88 PP with a EF86 to a 12AT7 PI.  No, nada nfb to be found simply because it sounds better.  This amp was built with serious Iron.  The 2 pieces of iron weighed 24 lbs and I expect 5 would be even more.  Basically it is a hopped up DR Z Route 66 except it will produce 80 watts with much more current and the KT120's see 570vdc.  This works well for him, but in the last few years Country Music has taken on some Rap, Dance and Pop styles with Jason Alden Guitarists both playing MESA Trip Recs and Humbucker PRS grinding much harder the Bad Company ever did.  Right now he is using pedals and I encouraged this.  Well sometimes folks just need something new.



Just the cabinet comparison will save him 60lbs, so he will end up lighter than he is now.



I know most do not understand wattage vs volume, but understand he is an exceptional player and knows feel and always turns his volume up to a breakup and uses 2 Tube Screamer, ts9's.  Compressor and EP boost and just like me, he doesn't like Chorus, Delay, Phaser......Modulation basically running through the front and mixing the signal.  We both use a WET/DRY setup.

[Ed_Chambley]

I know the customer is always right...but
if you got a BB, put together any PA tube pSE. drive them with anything you want.  now reconfig to PP self biased, repeat exact input.  tinkerin, I've found the "SE" sound comes out nearly as well as pSE, til 9, 10 there is a difference, anyway FWIW

I know what you are referring to.  Here is the problem with the comparison.  When I turn off one of the 5881's and only use one, the difference is much greater simply because the tube clips much sooner, and I know this happens because of having 2 smaller OT's because I tried it both ways.  Since the signal voltage doesn't change between 1 or 2 power tubes *I THINK* the 5881 and smaller transformer together get pushed 2 x harder.
Plus this amp I built I used Mid Century Modern style cabinet (he loves this type of thing) with a light beige tweed cloth that wraps half way and a Suitcase Handle and I am sort of neurotic with my amps so it does have a really good tone which a lot comes from an oversized ported cab with a single 10 Gold.  It does have a very thick tone, cleans up well and will sustain until the onset of feedback AND you can stand right in front of it without any bleeding in the ears running one tube. I am sure when 4 are running it will have more volume, plus we hear the second as musical. I will say a lot of players love this and he is no exception.


He was looking at SWART Amps, but the Atomic Spaceman Reverb or whatever it is called is a Reverb Champ basically and their drummer stage volume is like the old days.  IOW, he sweats, but this place is at least 75000 sq foot.  Maybe bigger.


Anyway, he does pay me well and is a close friend and knows it may not turn out like the tone in his mind, but he did say 2 of those Brownface Bassmans like I use would work.  That is until he saw I had some XF2's.  I have a few boxed matched pairs, but I also have a few that are not as closely matched which we will put in.  If it doesn't sound right, I will simply change output tubes and tweak the preamp.

[jjasilli]

I don't see the point, or any benefit, of having the 2X parallel SE power tubes connected to 2 OT's.  Rather it's a detriment, and not just because of needless extra weight, but because of the impedance mismatch @ 1/2 power.


IMHO: Run both power tubes into 1X OT, and cut signal to 1X power tube for 1/2 power.  Impedance will be preserved.

[Ed_Chambley]

I don't see the point, or any benefit, of having the 2X parallel SE power tubes connected to 2 OT's.  Rather it's a detriment, and not just because of needless extra weight, but because of the impedance mismatch @ 1/2 power.


IMHO: Run both power tubes into 1X OT, and cut signal to 1X power tube for 1/2 power.  Impedance will be preserved.

This is logical and what I planned initially.  Thanks!

[PRR]

.

[jjasilli]

Clever! But, why?  (Also may pop when switched)

[shooter]

seen this thread;
http://el34world.com/Forum/index.php?topic=24436.0
look at reply 7 schematic (PA), for your 1/2power idea?
can't find the other example, but it uses a string of "Christmas" incandescent lights, IIRC like 8, switch in 3, 5, all 8 for "stepped" attenuation.  You'd have to do it to both tubes to keep impedance, somewhat happy anyway, I'm still thinking 

[Ed_Chambley]

seen this thread;
http://el34world.com/Forum/index.php?topic=24436.0
look at reply 7 schematic (PA), for your 1/2power idea?
can't find the other example, but it uses a string of "Christmas" incandescent lights, IIRC like 8, switch in 3, 5, all 8 for "stepped" attenuation.  You'd have to do it to both tubes to keep impedance, somewhat happy anyway, I'm still thinking 

That is my preferred method. Samamp uses this and it is patented. You have to like some sag, but it works great. Then you need no master volume. VOX uses resistors in this manner, but the feel changes too much.


I love the lights to shine on a stage cause my eyes are not 25 anymore .

[Ed_Chambley]

.

Thank you, easy to understand.

[SILVERGUN]

The only fairly intelligent thing I can think to say is that I would consider doing the line out as a Herzog type of a resistor into a pot.
I imagine that most people have never had that luxury and I remember the first time I tried it it really blew my shorts up.


My opinion is that it is worth it. Whatever it takes to get to his SE dream is worth trying.
I love my SE power amps and have not thought about trying anything else since I built them. It is a great feeling to feel like you have found your tone and the search is over.
I built my power amps with two volume controls and I never use either one of them because they both stay on 10. I control my volume with my volume knob.


My other opinion is that 1 KT88 beats two of anything any day of the week. I run mine through a One Electron OT that I bought from DL. Its rated at 120mA and Im runnig it at about 130. I could pop popcorn on a good night.


Right now my hands are sore from sanding spackle but I can't wait to get back to my rig.

[shooter]

2nd everything that wise man said!!!

I never use either one

my last KT88 SE got downsized to 1 drive pot, and 3position tone knob
all the other knobs SHOULD be somewhere between wrist n elbow 

[SILVERGUN]

what I've figured out, bigger B+ is better, as you know.

The only reason I didnt say this is because shooter said it for me....'specially since you mentioned "stay clean"


I was surprised by how far my B+ was dragged down by burning up a single 88.
I didnt run a SIM but if I would have, I would have chosen a bigger PT.
I do remember shooter, DL and PRR trying to talk sense into me, but I already had the 290cx sitting here....one regret,,,low B+

[shooter]

I was surprised by how far my B+ was dragged down by burning up a single 88.

yup, I bought a new 88, put in a 2nd PT (same values) cuz I knew there was something wrong, but, it was me 
my notes now say "expect up to 60vdc drop from KT88's at 100% dissipation, BUT, it's worth it "