Champ and recommended load impedance

[Leevi]

I'm planning to use a Hammond 125DSE OT in a Fender Champ like amp where
the B+ will be ~330VDC or higher. Which one of the wiring alternatives do you recommend to use
with a 6V6GT i.e. 5000 or 10000 Ohm

http://www.hammondmfg.com/125SE.htm
http://www.ampwares.com/schematics/champ_aa764.pdf

/Leevi

[DummyLoad]

there's a discussion on said topic in link below... 

--DL

http://www.el34world.com/Forum/index.php?topic=11664.0

[Leevi]

Thanks for the link DL but I didn't find answer to my question.

I'll use 6V6GT as a power tube, where on 315V the load resistance is 8500 Ohm.
If I increase the plate voltage is the load resistance getting closer to 10000 Ohm?

http://boozhoundlabs.com/howto/pdf/6V6GT.pdf

/Leevi

[bigdaddy]

Lets look at things this way. Everything is dependent on something else in terms of what the actual value of anything is. Lets take a speaker, it says 8 ohms but what is it really? Also from what I understand the impedance is dependent on frequency. Also each transformer has a +/- of 10% or 20%, Hammond has been great but some of their stuff recently has not been up to their usual standards, me and another person both blew out some small PTs of theirs for a firefly, they should not have blown. I'm sure you could make pages and pages of calculations and it will come out with somewhere in-between. I think original Champs were 6.6K but they sound better with an 8K OT, IMO.

So what we have here is something I tried to contact Hammond about and they say that's the way they make them period. That's why when using a 6V6 or 6L6GT I used Allen OTs in my champs at 8K. So it really depends on the voltages as to which one to use. If you are going with the higher voltage it might be best to use the 10K tap. You can probably use either one so it also can be a matter of which sounds better, it might also affect your NFB too so that's a consideration with the sound/tone. It's not like you're messing around with vintage stuff and the older Hammonds are over built. Put a 4/8 ohm switch in and try A/B'ing them. Also what 6V6 are you using, the JJ as opposed to a more closer type to original 6V6 will sound and react differently.

I would use the 10K tap for 8 ohms. I removed the NFB in my champs anyway, they sounded way better without them.

[Fresh_Start]

The formula from that other thread seems to show 9,000 as a ballpark
(330^2)/12 = 9,000

GE & Tung Sol tube data sheet shows 8,500 for 315 volts on single-ended 6V6.

Allen Amps replacement (TO8C) is 8,000 ohms

Hoffman's 22905 is 7,000 ohms

The Hammond 1760C intended for a Champ shows a 7,000 primary with a 3.2 ohm secondary.  That translates into 8.750 with a 4 ohm secondary.

My guess is that starting with the 10K tap would make more sense with those voltages.  It'd be better if you could put in a switch, if only temporary, to HEAR the difference.

HTH

Chip

[Leevi]

I think I'll start with 10000 Ohm. Thanks for the replies.
/Leevi

[DummyLoad]

typically in a single tube A1 design, you run Pdiss @ 90% of max. rating. 6V6GT power rating in A1 service is 12W max. 11W is close enough to 90%, so we'll use that.  

to solve for Rload, we know what Pdiss max is, but... what's is your B+ to be?  300? 350? 400?

determine that, then apply formula. Va² x Pdiss. (11W in our 6V6 case).

backwards... example you have a tranny, it's 9K into 8ohm. i want to run 6V6GT, A1 single tube with an 8 ohm speaker load - how much B+ (Va) do i need where 9K load is optimum? consult ohms law pie chart. we have R we have P we need to solve for V.

sqrt p x r = sqrt 9k x 11W = 99K x sqrt = 314.6V

this is a simplistic approach but it gets you in the ballpark and works.

in reality, if running cathode / auto bias, you would subtract Vk from Va then that would actually what you'd use for value of "V". you're solving for the power dissipation of the tube only - not the tube and the cathode resistor. in other words, power dissipation of tube is the voltage measured from anode to cathode * the current flowing through the tube; the voltage measured from anode to cathode usually called Vtube. if you were to run fixed bias, there is no cathode resistor so then Vtube is measured from anode to gnd.

[PRR]

BSE or CSE would be plenty big enough for one 6V6.

[Leevi]

BSE or CSE would be plenty big enough for one 6V6

Yes, but you can get the DSE almost for the same price as the smaller ones. ESE is then more expensive.

Model   Price index
125BSE 100
125CSE 107
125DSE 113
125ESE 150
125FSE 195
125GSE 237

/Leevi

[Fresh_Start]

BSE or CSE would be plenty big enough for one 6V6

Yes, but you can get the DSE almost for the same price as the smaller ones. ESE is then more expensive.

Model   Price (units)
125BSE 100
125CSE 107
125DSE 113
125ESE 150
125FSE 195
125GSE 237

/Leevi

I believe that PRR was telling you that bigger does not automatically equal better, especially with output transformers.  I put an "upgrade" OT in my 5F2-A bastard and lost some of the grit/mojo that I really liked about the amp.  Also gained more bass which the amp doesn't need for my purposes and with my speaker/cab combination (10" speaker in Tweed Princeton/Harvard sized box).

IMHO the output transformer is one of the most important components in terms of the amp's overall tone.  Less important than the speaker, cab, output tube(s), but maybe more important than your choice of preamp tubes.  If you're looking for a simple 2-knob amp with lots of clean headroom and strong, clear bass, to drive a 1x12" cab then by all means get an over-sized OT.  A 6L6 might be a better choice for the output tube too.  OTOH, if you want classic Tweed Champ/Princeton touch response and grit/growl, an UNDER-sized OT might be a better choice.  It all depends on what you're after.

HTH

Chip

[Leevi]

It all depends on what you're after.

Yes, there is probably not a right answer if the OT should be under- or over-sized.
I have mainly followed the rule "the more iron the better" but have also been
satisfied with some amps where I have used under-sized OTs.

[bigdaddy]

I experimented a lot with those transformers and champ type amps.

Bigger is NOT better, not at all. More is less. It really depends on the tube and a 6V6 does not like huge transformers at all. The stock champ OT is a joke and is too small but much bigger does nothing for the sound at all. The best sounding transformer I used for a 6V6 was the Allen TO8C. I tried bigger OTs and they sounded horrible with the 6V6, a custom Heybour they made for me and a Doberman which was huge. By using those huge OTs the 6V6 sound was lost. What makes the 6V6 so good is it's tone. The tone of a good 6V6 amp is it's sweetness and smooth warm distortion. Two ways to lose that is to use high voltages and huge OTs.

[Leevi]

Maybe I should learn from you Bigdaddy.
To select a smaller OT is not problem, but what is the B+ you recommend for a 6V6?
/Leevi

[bigdaddy]

I tried a lot of voltages and 6V6 tubes and even the JJ which can take a lot sounds best below 350v. Between 300 and 350 sounds best with a 6V6. That's were you get the most of what makes a 6V6 such a great guitar tube.

I also think they sound best without NFB. The lack of NFB makes up in gain for the lose when using a full tonestack. It works out well. I sold a few of the Champs I built on ebay and everybody thought they sounded great. It was a simple champ no NFB, full tone stack, the TO8 and a JJ 6V6 the voltage was around 350V on the plates.

[Fresh_Start]

Honestly, I liked around 300-320 best in my 5F2-A and experimented a LOT with B+, but YMMV.  That's with an NOS 6V6.

Chip

[phsyconoodler]

I used a 272HX Hammond and a 5Y3 with a pair of 6V6's in parallel and a 125FSE OT and the amp sounds fantastic.Lots of chime,killer breakup,loads of headroom and generally kick-ass tone.With a 5Y3 it has 330v and I use a 5AR4 and pop in 6L6's or KT66's and the amp is lovely.It is far from lifeless and I don't think a smaller OT would sound as good in this amp.i fiddled a lot with load and ended up using 8k I think.The beauty of the Hammond is you can switch it easily till it sounds best.

[DummyLoad]

first guitar amp buttery and i built was a 2 x 6V6 in A1 "single ended". it started out as single 6V6 with a 5K load hanging off it, but soon became a 2 x 6V6 stage into 2K5 load. we tuned Rk to keep the dissipation in check. with 2 6V6 we ended up with 2 x 330R in parallel or simply stated, about 165R for 2 tubes. looking at my notes, Vk is 16V over 165R; so two tube are flowing ~97mA and dissipating 34 watts.

B+ is around 360V w/ two tubes and 370V single tube. it uses a 6SL7 and 6SJ7 front-end. it is loosely based on a valco 1948 oahu amp. we added a pentode front-end and hot-rodded the output stage.

just about any reasonable tube load that the hammond can be configured for will work - optimum is ~8K for ~300V. take this case in point that 5K will work, and to our ears, with a single tube, 10K sounded awful, since we had no option for 7-8K load we used 5K and it sounded best. we wanted more power - we added another tube and halved the load to 2K5.

the amp ended up gathering dust in a corner (neglected in the pursuit of nirvana) so we traded it to one of buttery's electricians to make more room - he's still rockin' it. to me, even w/ 2 x 6V6 it was never loud enough and hence why i pursued the high(er) power champ-amp route. i wish we hadn't traded it away now.  
 

--DL