HELP design of switchable SE amp el84/el34 ?!

[FatGaz]

HI,

I was having a clearout / tidy-up and found an old amp project that I abandoned - SO it's time to re-build it to something new.

I'd like a single tube SE amp for quieter growl, but I might also need it to do clean sometimes - THUS i'm thinking of having an output stage with two tube sockets, one for an el84 and one for a el34 - at the moment I'm prepared to just swap tubes in and out as needed, leaving one socket always empty.

Anyhow, I need help with designing this !

I have been thinking of trying to use a shared power supply to the plate and screen, so the same voltages, and separate cathode bias resistors. As I understand it, something around 250-280V might be OK for both an el84 and an el34 ? OR should I be trying a different octal tube, maybe a 6L6 ?

I'm not good enough to work out the correct bias resistors, I understand that this it dependant on the type of tube and the plate voltage it's got ?

also, I'm thinking I'll put separate grid resistors and screen resistors on each socket ?

I'm not sure about the preamp output voltage levels - since I'm thinking of dirty/clean, I thought that feeding the same to both tubes might be ok ? or should I reduce the voltage to the el84 to even things out a little ?

SORRY I have asked so many questions, but I don't have an example amp schematic to copy / clone - thus there are so many pitfalls to making an amp that sounds bad.

(ps I'm thinking of getting a hammond 125ese OT - should be OK I think ? - i have heard bad stories of impedances being wrong and OT's arcing etc.... )

GARETH

[MWJB]

For EL84 & EL34 you're going to need a decent 6.3VAC supply, 3A at least, preferably more like 4A. If you really want to go ahead with this then I'd think about upping the B+ to just over 300v.

Really though, I'd be looking to use 2 slightly more similar tubes voltage & impedance wise, like EL84 & 6V6? If you compromise too much on the octal tube's voltage it won't be really any cleaner than the EL84. Again, if you raise voltages on the EL84 to compromise with a bigger octal tube, the EL84 may become to sterile & strident, even drowning out the other tube.

For really mismatched tubes, I think a stereo output would be better...but that complicates things somewhat, still requires a reasonable compromise on plate voltage, though obviously you would need a B+ dropping stage between the octal & EL84 OT centre taps, but this could work in your favour. OTs would need to be mounted at 90degrees to each other.

Or, you could still use 2xEL84 wired "self-split push" (SIPP) pull for your cleaner sound, switching to 1 tube SE for low volume/dirt. Should be feasible using a switch that selects a single cathode resistor for one tube, or a common/parallelled cathode resistor for 2 tubes. If you wanted it to be real simple, compromise on the single cathode resistor & just pull the out of phase tube, though a lack of air gap in the OT could lead to the OT becoming magnetised from running SE (wouldn't really worry me, PP OTs work well enough in SE applications for guitar)? Then your transformers wouldn't need any compromise on B+/primary Z.

Last option, go even simpler, 1x EL84 in SE, obtain the different voicings through switchable preamp voltages/cathode resistors/cathode bypass caps etc.

[FatGaz]

Hi, MWJB lot of interesting ideas, thanks.

I'm entirely not sure if you understood my idea though, I was going to only ever use a single output tube, either the small el84 and hit it hard OR a larger octal tube for clean headroom. I don't envisage using both together as either push-pull or parallel SE. (I may have misunderstood your post instead !)

not sure i'd like to risk the Self-split to SE idea, I guess it might work, but i'd need an oversized OT ?

you have sparked a thought, though. i wonder if running 1 or 2 SE parallel tubes would give me the clean option ? yes I'd need to remember to use a different speaker tap to get the impedances correct.

I could put a switch in to lower the B+ for using the different tubes ? I could put series zenners in the ground of the rectifier to do this and swtich them in/out ? (with the amp turned off)

IS any of the above a good or a bad idea ???

thanks,

Gareth.

[MWJB]

No, I understood the post, I was trying to point out that subbing a EL84 & EL34 in the same circuit is a complicated issue to address & that a EL3 run at typical EL84 voltages may well sound dull & stodgy, failing to deliver the clean option that you were looking for.

Self split would require a PP OT. I don't see any "risk", might get a couple of goes to perfect the switching (don't know how much the tube would like having the grid grounded & cathode open with full plate supply?, Maybe rig the switch to add 2nd cathode resistor & lift out of phase tube grid?), but then so will any of this. Hint - Google Victoria Ammo Amp.

Running 2 parallel tubes would give more headroom than 1 tube, but a PP option would give significantly more cleans.

Zener switch is a workable idea...if you accept the fact that you will one day switch it to high voltage with the low volltage tube installed! ;-)

[FatGaz]

No, I understood the post, I was trying to point out that subbing a EL84 & EL34 in the same circuit is a complicated issue to address & that a EL3 run at typical EL84 voltages may well sound dull & stodgy, failing to deliver the clean option that you were looking for.
<SNIP>
Zener switch is a workable idea...if you accept the fact that you will one day switch it to high voltage with the low volltage tube installed! ;-)

Well, I could put the zenner B+ switch next to the tube sockets or PT, and not on the front panel... (i was thinking of having an on/off/on standby switch with the two voltages - but this would defo. risk setting it to the wrong voltage, as I am absent minded)

can you personally recommend sonically-advantagous or preferred voltages for the EL84 and EL34 when run single SE ? (or point to good amps/ schematics) I have looked at the AX84 site for example. I'm not too good trying to select bias resistors from load lines.

would I be better trying a 6L6 rather than a EL34 ?

thanks,

Gareth.

(the reason I want flexibility, is that this is going to be my only 'combo' amp.)

[MWJB]

An EL84 running at 250v+ plus might make 6W, as will a 6V6 running around 400v...you're not going to get much more than that out of a 6L6/5881/EL34 in cathode bias (400-450v at the plate to ground, 8W-ish)...so flexibility in terms of power/loudness is pretty well a moot point. Tone & envelope will change with tube type though.

Commonly used cathode resistor values are 150ohms for a single EL84 at around 250vdc from plate to ground, 220ohms at 300v-ish. 330-390ohms for a 6V6/6L6 at low 300s, up to 470ohms at 400-ish, 680-800ohms for anything much in excess of 400v. For the octal tubes use a 10W resistor, bypassed by a 100v cap. 2W resistor, or better will do for the EL84, 25v+ rated cap. Bypass caps should be mounted away from the resistor body.

EL84 & 6V6 are often biased to max plate dissipation in SE (often somewhat over), bias the larger octal tubes to 22-23W.

I really don't like the zener idea unless a compromise is reached where the amp will operate safely at both voltages, with either tube.

SE amps struggle to hang with a drummer using sticks, even though a simple PP option like self-split might not result in the same power as a phase inverter driven PP, it might just give you enough of a leg up for lower volume gigs. I think that this, or keeping to a single tube (or tube group type e.g. EL34/KT66/6L6/5881 - wire socket for EL34 use and all the others will "drop in") with other features (like cathode/fixed bias switch, attenuator network & aforementioned mods) would be a pretty flexible SE amp. Many of these features are already used on commercially available amps.

My favourite option would actually be a stereo champ style amp, octal power tubes, fixed bias, in a V front 2 speaker cab, then you can have 6W/12W depending on which input you plug in to & with decent speakers you can listen to your CD's/I-Pod through it too (if it's pretty enough the wife might let you keep it in the front room, "Hello dear, I thought I'd build something useful for the home - a stereo CD/MP3 player!" & you can just grab the axe whilst she goes shopping/soaks in the bath etc)...'cause that's what I'm going to do!

[FatGaz]

An EL84 running at 250v+ plus might make 6W, as will a 6V6 running around 400v...you're not going to get much more than that out of a 6L6/5881/EL34 in cathode bias (400-450v at the plate to ground, 8W-ish)...so flexibility in terms of power/loudness is pretty well a moot point. Tone & envelope will change with tube type though.

if i understand you, a SE EL34, when cathode biased, might not give me much more clean headroom than a EL84 cathode biased SE ? I'm a little supprised at that, or is this more a function of the similar plate voltages ?

maybe a parallel SE EL84 is the easiest 'clean headroom' way forward, if i want more flexibility ?

thanks for the advice and thoughts,

Gareth.

[MWJB]

"I'm a little supprised at that, or is this more a function of the similar plate voltages ?"

Well the EL34 might run 150v more than the EL84, it's more because the EL84 is a very efficient tube, simply put it punches above its weight, even at the lower voltage...there will be differences, more low end on the EL34, improved dynamic response, but not significant differences in outright power/perceived volume.

[guitarmike2107]

I have built several dual SE amps with EL84 and Octal sockets, B+ was ~315v. sounded good.
Check out the cornford carrera, which is an amp that has both sockets available.

You could make the b+ switchable between a low voltage for the El84 and a higher voltage for the Octal.

I agree that this type of amp will struggle to keep up with a sticks drummer banging it out. But it is also way to loud when cranked for most apartments/houses in the uk. Unless you live on a farm or something

VVR is another way to solve that problem though.

You could also build a dual SE amp with the EL84 and 6v6 as suggested, and have a mix control to determine how much each valve contributes to the output.

To many ideas and options…..

[FatGaz]

I have built several dual SE amps with EL84 and Octal sockets, B+ was ~315v. sounded good.
Check out the cornford carrera, which is an amp that has both sockets available.

You could make the b+ switchable between a low voltage for the El84 and a higher voltage for the Octal.

Interesting , thanks for the input, did you try EL34s or 6L6's ? Did you do anything for the significantly differnt 'sensitivity' of the EL84 and EL34, or did you use this as a feature ?

VVR is another way to solve that problem though.

VVR ??? what's that ?


ALSO, one thing that I'd like to do (but a bit off topic) is to add an attenuator, It seems you need to make sure that the impedance seen by the tubes doesn't change too much, in other words L-pads etc. should be roughly 16 ohms for the 16 ohm tap.

I've played both a vox ac4tv - nice and small but badly made- has a L-pad attenuator for an el84 SE, - and a cornell romany - a 6L6 se i think - but no idea how the low power thing works - I'm not sure it's a resistive/L-pad arrangment from the chassis pictures iv'e seen. it does go very quiet on the lowest setting - expensive but very well made.

thanks All !

Gareth.

[MWJB]

Low power mode on the Romany's I have seen was not achieved with a speaker attenuator...may have been updated since then?

[FatGaz]

Low power mode on the Romany's I have seen was not achieved with a speaker attenuator...may have been updated since then?

Do you know how it's done ?, I was impressed by how well the "attenuator" worked when i played it; i think it had four settings. it's a lovely amp, classy looks, will last forever, but I can't really afford it at the moment. ( and I have 3/4 of an amp allready, but in bits !)

is it just a reduction in voltage ? (maybe selecting taps on the pt ???)

G.

[FatGaz]

just a quick question as i need to mailorder some iron... (asap)

I;m not sure if a hammond 125ese is big enough to cope with an el34 ? it's rated as 15W 80mA. I note that ax84.com 's projects use the 'e' with the el34.

the next size up is the 125Fse rated at 20W and 90ma - but it's a lot bigger/heavier.

I only want to buy an OT once, and I know the bigger one is les risky and will work, and I guess it might be enough to cope with 2 EL84's in paralled se if I go down that road as an option?

cheers all ! and thanks for all the help.

Gareth.

(i'll have to take a few photo's of the build, so you can laugh at my poor diy skills)

[guitarmike2107]

I only want to buy an OT once….

No chance, once you build one you will want many many more. It’s a disease, welcome to the asylum.

125ESE is fine for a single EL34/6L6 type amp, some have even used them with KT88. You can’t run two parallel EL34’s into a 125ESE or a 20watt 125FSE unless you restrict the amount of current to both valves to under 80/90mA respectively. i.e. bias both tubes cold.
Bigger and heavier is great for Bass but more expensive. But this isn’t a commercial amp so …..

VVR is a variable voltage control for lowering the voltage to the amp, it uses a mosfet and generally seems to be the best option for lower powered amps. I would say don’t do the L-pad option as at lower volume settings it really doesn’t work well. If you want the l-pad option build it in a separate little box.

Using different taps on the PT for different voltages also works well, but you will likely have to get a custom PT made up… not that expensive really.

I have used EL34, 6l6’s, 6V6 and KT77 in se octal builds, They all sound different, the SE amp I play at home has an EL34 in it, but I also really love the sound of the 6L6’s for slightly cleaner stuff.

cheers

Mike

[FatGaz]

125ESE is fine for a single EL34/6L6 type amp, some have even used them with KT88. You can’t run two parallel EL34’s into a 125ESE or a 20watt 125FSE unless you restrict the amount of current to both valves to under 80/90mA respectively. i.e. bias both tubes cold.
Bigger and heavier is great for Bass but more expensive. But this isn’t a commercial amp so …..

I was thinking 2 EL84's parallel vs 1 or 1 octal - I think 1 EL84 is ~48mA so the 125 E se might not be enougth - or just bias things down a bit as you said to be around 80mA total ?

VVR is a variable voltage control for lowering the voltage to the amp, it uses a mosfet and generally seems to be the best option for lower powered amps. I would say don’t do the L-pad option as at lower volume settings it really doesn’t work well. If you want the l-pad option build it in a separate little box.

OK, like the london power 'power scaling' - i think you can get cheap kits... SO I could also 'brown' the amp B+ line by the zenner trick on the bridge rectifier ground ? I assume that you need to keep the plate and screen voltages correctly scaled, while the pre-amp doesn't matter for power output.

I'll look into it as a possible modification after I've got the amp working. I'll try to leave space in the chassis.

I have used EL34, 6l6’s, 6V6 and KT77 in se octal builds, They all sound different, the SE amp I play at home has an EL34 in it, but I also really love the sound of the 6L6’s for slightly cleaner stuff.

COOL, I might get both an EL34 and a 6L6 to play with then.

thanks for the advice

Gareth.

(biggest challenge I think is getting the chassis to fit with a 12 speaker in a combo - the chassis was originally for a head. AND my woodworking skills are rubbish... I did score a pair of used 70's G12H's recently, so keen to find a home for them.)