voltage on G2 higher than A on el34's?

[black_labb]

i've just bid on some old transformers on ebay from a guy who put up quite a numeber up for auction, and started to plan what i would do with them. looking at datasheets, it seems that el34's are suggested to run with Grid 2 voltages higher than the anode. it seems that higher anode voltages become suggested when you have voltages 450 and higher (on whichever is higher). having looked at some schematics for guitar amps, i havent seen any amps with g2 higher than the anode. is there any reason for this?

im hoping for an amp that will run either 2 or 4 el34's, and the power transfrormer has a 300-0-300v secondary, which gives about 420v with diode rectification. it seems i could get ~55w per couple of el34's with ~425v on g2, and ~400 on the anode. what kind of output level would i be able to expect from 420v on the anode, and what would you recomend for the voltage on g2?

if i were to do the voltage on g2>A, would i just hook the B+ straight out of the rectification to G2, and the ot centre tap to the connection after the choke on a typical power supply? or would g2 need more smoothing of the powersource for proper operation? (if this is the case then it makes sense that i havent found an amp with this style, as that would be a hassle having an extra choke).

it just occurred to me that the output transformer acts a bit like a choke for the b+ at the input on a typical design, is that right?

the amp is for bass firstly i think (depending on wether i get the big power transformer or not), so i want atleast 75w from 4 el34's. i already have a 200w valve amps for bass, i want an amp that i can crank a bit to get an overdriven tone at lower volumes/without worrying about blowing speakers. the amp will probably be similar to a hiwatt in terms of the preamp/PI so the amp should work well for guitar as well.

thanks for any help

luke

[abro163]

Never heard that before normally g2 is lower than anode

But if you look at a fender 5e7 they connect the transformer centre tap and screens at the same B+ tap, so you get slightly lower voltage at the plates than the screens, because of the transformers resistance.

However the choke has to handle the entire plate current.

[black_labb]

http://tdsl.duncanamps.com/show.php?des=el34

theres a link, i also checked a phillips datasheet which gave more or less the same results. following the list of configurations you get g2>A until the anode voltage reaches 450, where g2 looks like a more normal value in relation to the anode.

just had a look at the 5e7, and your right. not really the same thing, as these values have the voltage 25v different, where the drop on the 5e7 would be a few volts or so.

[MWJB]

Almost every EL34 amp you will ever see runs a 1K 5W screen grid resistor to each EL34.

marshall 1962 bluesbreaker runs 420v-ish at the anodes & is rated at 30W. Perhaps a shade conservative but 55W RMS from 2 EL34 at 420v seems a little optimistic.

Expect to lose a few volts off that 420 when the PT is loaded with tubes.

6550 or 7581A (perhaps even KT66) may be a better choice for bass. 1K 5W at the screen grids will work fine with any of these.

A choke input filter (5E7 style) is probably not the best idea for a bass amp. Even if you did go thios route, I'd still incorporate 1K 5W screen grid resistors.

[black_labb]

i see what your saying. im not really looking for the 55w per pair, but 40-50 would be nice. the 55w was the value from the datasheet, not my aim. have a look at my link to see what i mean.
im curous about the voltages on the datasheet. it must be a valid way to get some more power from el34's witout upping the voltages, otherwise the configuration wouldnt be on multiple datasheets. im wondering how well it would work for a guit/bass amp.

i'm not planning on a choke inputfilter, was just an observation made by abro163 that seemed relevant.

[MWJB]

Few guitar amps match the suggestions on tube data sheets.

"it must be a valid way to get some more power from el34's witout upping the voltages". Upping the voltage is the easiest way to get more power. For bass this would probably be the better route anyway, say 470-500v on the anodes (345-0-345VAC to 375-0-375VAC)?

[Steve Conner]

The more voltage you put on the screens of EL34s, the more power you get out of them. You can think of the screen voltage like the boost on a turbocharged car, with about the same risk of disaster if you turn it up too far.

In some old experiments I did with a 6.6k OT, I got 50w exactly at 475V B+ and 360V screen supply (through a 1k shared screen resistor.) When I upped the screen supply to 475V too, I got over 60w, but a strange light came shining from inside the plates. It was the screen wires glowing like lamp filaments. Your call as to whether this is good for the tubes. I decided it wasn't and switched to KT88s, getting almost 80w.

In the tube datasheet, when you see Vg2 higher than Vp, it's because the voltage drop across the OT's DC resistance just happens to be more than the voltage drop across Rg2 at idle.

[black_labb]

the tranny has a 300-0-300vac secondary, i was hoping to use this for the amp. im trying to scoop up these trannies for cheap, as getting a big enough tranny in australia is expensive, and being a student, a pair of new trannies for a project like this leaves little for other projects (and i have 3 months off between semesters, so i have time for quite a few others). when you say that 470-500v would probably be better for bass, is that because there tends to be more headroom at higher voltages, or another reason.

im happy to have less headroom with this amp. i want to put quite a clean preamp in and overdrive the output tubes, so less headroom is not a bad thing. that's one reason why i would prefer el34's to 6l6's, which probably match up better with the pt.

extra after seeing steve connor's post: that makes sense steve. so really the plates are pulling alot of current, dropping the voltage quite a bit, where the screens dont draw much at idle. as the stage amplifies a signal the screens will pull more, and their voltage will drop more? when you say you increased the screen votage, was this just using different screen resistors? would this not be a bad option for me, as long as i keep within acceptable screen voltage levels (and watching for issues like you mentioned). being limited to 420v, staying within specs shouldnt be too difficult.

[Steve Conner]

Headroom is power. Specifically, the difference between the amount of power the amp is capable of before clipping, and the amount you're actually using.

More voltage means more power, so more headroom.

You wanted something overdriven, so you wanted less headroom. Zero or negative headroom, even. Therefore, buying low-voltage PTs was probably a good idea: you'll get a low powered amp that needs overdriving to be heard in a band context. Probably about 35-40W, which was what I got when I tried EL34s on 420V. 40 with a 470 ohm Rg2, 35 with a 1.5k.

So just stick it together and see how it sounds.

[black_labb]

sorry, headroom probably wasnt quite the right term. i've heard that lower voltages tend to create a more gradual onset of distortion, or something around those lines. 35-40w per pair sounds more like what i'm looking for. the 30w from the bluesbreaker seemed a bit lower than i was interested in.
there are 3 different pt's that the guy is selling, one set of 3 260-0-260vac(probably good for a pair of el84's, one 300-0-300 that isnt too huge (2x6l6 or el34, maybe a quad of 6v6's), and another 300-0-300 that seems very big. he says it would handle a twin or similar easily. theres also a ot with a 3500Ohms impedance ratio that he says is good for 30-40w. the ot would probably pair up nicely with the smaller 300-0-300v tranny. either of the larger pt designs could end up with el34's, it depends what i end up with.

im hoping i get atleast something. theres also another ot with unknown ratio may end up bidding on that one as well, but i'll see what comes from the pt's, as they end a day earlier

[MWJB]

Do you have current ratings for the PTs (B+, 6.3VAC secondaries)?

[black_labb]

no ratings as they are out of old amps, judgements are based on sizes (not the best way, but they should be worth a try)

[MWJB]

Be aware that wall voltages have risen ove the years, depending on age you may actually see more like 330-0-330VAC? Also be aware that if current ratings are borderline you may see a significant driop in actual B+ when the P is loaded up.

Can you not source Hammond transformers ata reasonable price?

[black_labb]

prices here are quite high for hammond (or any other production part). it tends to be cheaper to import, but the australian dollar has really fell in the last month (went from ~.95 american to 0.60, its about 68 now). these transformers may well go for very modest prices, and im happy to buy these, and down the track get some new ones if these dont work out.
i've been planning on a 80-120w amp for a while, and if these dont work out/i get outbid then i'll be getting something similar from a hammond supplier.

edit: heres the big one, the others are from the same seller.

http://cgi.ebay.com.au/ws/eBayISAPI....m=370107413714