bet you guys guys thought i had given up!!

on the contrary, some semblance of a victory:

powered up by bypassing the mains selector - DC voltages looked good everywhere. im not 100% exactly the range i should be seeing on all the various pre-amp tubes, but suffice it to say all the plates had high-ish voltages and all the cathodes had low-ish voltages, except for the PI which had about 80VDC at the cathodes. power tube voltages looked good, with about 450V on the plates and -50 on the grids with the bias turned all the way down (with tubes pulled at this point) - i put in a lil sine wave and traced it through and, sure enough, on the output of the PI i had about 80V p-p from about 100mV on the way in. now the preamp tube configuration in this amp is new to me, and the order is not intuitive (the PI is the second tube in....even though on the scheme its V5....!), so i might end up having to come back here if there is a problem.

popped the tubes in and checked the see if the bias circuit was working and sure enough it was! i had the light bulb limiter in at this stage and it started to get brighter as i increased the control grid bias voltage. that seems right - tubes are actually pulling current so the amp must supply it. wondering if even trying to bias them with a limiter in-line is a dumb move, but i am still nervous something is going to blow up.

anyway, put in sound and sound came out! (i was worried the OPT might have been bad)

so ill try it out tomorrow with a guitar and we'll take it from there.....

i have a question about this dummy load and DIY dummy load construction in general.




we have ten resistors in parallel in series with ten more resistors in parallel. how can their power dissipation be additive if all of the current flowing through the load has to pass through the first ten resistors before it passes through the second ten? meaning, wont the first ten resistors have to bear the full brunt of the power, anyway? what am i misunderstanding here?

let's say 72 watts going in to 8 ohms (and lets say these are all 40 ohm resistors to make it even and easy)

P = I^2 * R
72 = I^2 * 8
9 = I^2
I = 3A

that 3 amps splits ten ways as it enters the load (im ignoring the rectifier/fan cause my dummy load doesnt have one), so 300mA into each resistor of the first parallel combo:

P = I^2 * R
P = 0.3^2 * 40
P = 3.6 W each resistor
x10 = 36 W dissipated by the first group and the same by the second group
x2 = 72 W

now it makes sense to do it this way rather than just having one row of ten 80 ohms resistors, because in that case each resistor would be burning up 0.3^2 * 80 = 7.2 Watts instead of only 3.6 W.

the math makes sense to me. what doesnt make sense is that the series current (ALL of it) passes through one group and THEN the next. seems like it should ALL be dissipated in the first group seeing as it ALL passes through there. how does the current know there are ten more resistors to go? i guess the relationship here between voltage current resistance and power cannot be quite intuitive....

and say i built a 4 ohm dummy load with one row of ten 25 ohm 10 W resistors and one row of ten 15 ohms resistors and put those two rows in series (strictly hypothetical ) and i wanted to make this into an 8 ohm dummy load and i just added a 10 W 4 ohm resistor in series with both rows, right at the front or the end, that would be a bad idea:

let's say the same 72 watts, so 3A coming in -

that 3A passing through a row of ten 25 ohm resistors would only dissipate 0.3^2 * 25 = 2.25W per resistor, but when it passed through that added 4 ohm resistor it would dissipate 3^2 * 4 = 36W and bye bye resistor and dummy load and maybe OPT....

i guess after doing all this math i understand what's going on pretty well. the current itself only depends on total resistance, but the POWER dissipation depends on total current AND on how that current is divided/passes through the load.....

i think it may have burned up because i flipped the amp on without waiting for it to warm up when i was burning the tubes in - could the surging power/current have done the resistor in?

k all good.

put in a ceramic socket.

all voltages look good.

EDIT/FOLLOW UP: on the blues deville schematic, it says 13.6VAC on the output. wouldnt that only be giving me 20 something watts output power into 8 ohms?? (as per the schematic.....)? yet below that it says 58 watts rms.....can anyone clarify that test point for me?

amp sounding good and loud and punchy again. i think i maybe like the clean channel more now than i did - there is still something about the drive channel that i think i didnt really notice before, but now that im listening to things a bit closer than i used to, i realize the drive is a little bit character-less and even has a blown out sound. cant quite tell if its an actual issue or just the overdrive's natural character....

i might be back asking about that in a bit, but for the time being at least the amp is back to full power and doing what it should!!

thanks!

alright, we're good to go:

there had been a mysterious knob on the back panel of the chassis that i was just ignoring - that was the presence knob, rewired just like the schematic.

i removed the weird master volume mod and rewired the normal presence knob back to stock (left that extra knob on the back disconnected).

got some 6550s and put them in and biased them (they biased right no problem). one of the screen grid resistors blew up while i was burning them in, but i replaced it and things seem to be fine. thinking it was just old and ready to go.....

everything else seemed to be fine. amp sounds great! loud and really detailed sounding, with all kinds of surprising harmonics that come through with dynamic playing. especially once you pass a sweet spot on the volume and things start to break up. it was putting out about 13-14 Vrms across 4 ohms (so almost 50W) - i think we're all good!

thanks, guys - another amp back to life. and i learned some stuff!

oh so in the first half of V2, that anode goes to the grid in the second half of V2? is that right? so there is voltage gain there to feed the cathode follower?

didnt realize that tube was two stages.

QuoteTypical conditions for 6CA7/EL34 are;
54W out, 3.5k plate-plate, typical load ZL = 8 ohms, meaning an impedance ratio Zp/Zs of 437.5:1, from a turns/voltage ratio Vp/Vs of 20.9:1.

where do you get the 3.5k?

Quoterespective steady-state mid-band AC voltage measurements between primary and secondary can do that.

whats that?

as for voltage followers, i do understand the role/function of a buffer. what i dont understand, is why my measurements seem to give me gain across the cathode follower.  1 V p-p in (from anode of the first triode) and 30 V p-p out (off cathode of the cathode follower)

yes - the mod on the PI output has a direct impact on the output voltage. i think i will restore it - is it some kind of strange effort to put a master volume on the final stage?

it has an output selector. it was set to 8 ohms originally but i only made a 4 ohm dummy load (what a dummy) and so ive switched it to be on 4 ohms. should i trust the selector? if not, can i measure resistance on output transformer leads to verify?

Quote(except the cathode follower feeding the tonestack of course, that will be close to unity).

should that stage have been giving me unity gain?

okay:

putting in a 1khz 100 mV sine wave into the input (with no power tubes installed) -

stage one (measured at the anode): 2 V p-p
stage two (measured at cathode): 45 V p-p (with volume cranked)
stage three (measured at anodes): 100 V p-p (starts to clip)

should stage two be at unity? overall this seems like okay gain, right? about x1000. thats a big swing off the PI, right?

putting a 1khz 1 V sine wave into the treble control (no power tubes) -

stage three (measured at anode) 100 V p-p (starts to clip) - about the same.

putting in a 1khz 100 mV sine wave into the input (with power tubes installed) -

stage one (measured at the anode): 1 V p-p
stage two (measured at cathode): 30 V p-p (with volume cranked)
stage three (measured at anodes): 75 V p-p (starts to clip)
voltage at the 4 ohm load: 35 V p-p


this is with the not properly biased el34s in. i have some 6550s i will put in later on (a friend is bringing them). seems like the amp might be in okay shape, overall though, right? maybe just needed the preamp tubes moved around (which i did).

also i rechecked and yes, i get about 0.7V at V2 pin 3. but all the other DC measurements are the same as i posted previously.

strange detail: the presence knob has been re-wired on this amp. instead of being attached to the grid of the PI, it is wired as a rheostat between the two 22nF caps on the PI anodes. if this knob is all the way down, i cant get nearly as much unclipped power out of the PI as I can if i crank it up. what is this modification?

yeah i think its set up for 6550, according to the bias network.
but still, can anyone explain why two different sets of el34s would be allowing such drastically different cathode currents?

DC voltages:

V1:
pin 1: 100V
pin 3: 0.7 V
pin 6: 140V
pin 8: 1.4 V

V2:
pin 1: 120V
pin 3: 0 V (!)
pin 6: 220V
pin 8: 129 V(!!)

V3:

pin 1: 200V
pin 3: 34 V
pin 6: 200V
pin 8: 34V

injecting a signal straight into the treble control i can still only get about 60V p-p off the PI, though....doesnt this bypass V1 and V2, anyway?

k so i got new el34s, popped them in, but everything is *not* better.

1) even with the bias voltage as high as it will go (about ~30VDC), the tubes are only pulling 30 mA or so idle current. if i swap the old tubes back in, they pull way more. ??. the amp chassis says 6550 on it, and ive read that these amps were kind of made for those tubes. is there something i need to change in the bias circuit for el 34s to work? if that's the case, though, can someone explain why the old tubes (also el34s) could pull so much more current? but still make low power for the amp....?!

2) measuring at the plates of the PI, i only get about 40V p-p. that is definitely less than twice the bias voltage. ??

3) ive got about 400 VDC on the power tube plates and pin 4s. right heater voltage, too. no other suspicious stuff that i can see.....

Enzo!!

my god what a mess - pin 6 of the PI on the tube socket itself is *somehow missing* the actual metal part that the tube pin is supposed to make contact with!

i tried a few rigs but the only one thats worked, i think, is a blob of solder on the tube pin just so it reaches up to the connector.

and, voila, i get 33 v on the cathodes and about 240V on the plates....

i dont want to run the amp like this, but now i know that this is the problem....

now i assume ill just have to change that socket but i really dont want to do that! what a pain. any success stories of rigging something up in a situation like this? or has anyone ever encountered a situation like this?! i tried some metal molex like connectors but its just not cutting it.....

Quoteat the onset of clipping

yeah i turned it up until the sine wave started to wonk out a little bit.

the PI: you mean the AC voltages coming off the plates to feed the power tubes? these should be close to the DC bias voltages on the power tube grids?

so im trying to put some of my new skills to use -

someone gave me this amp to look at  - "sounds quiet"

B+/plate voltage - ~385VDC

tubes are EL34s
i did a quick bias, trying to get 14 watts or so dissipation.
started with one tube - looking for about 37mA. got it there with about ~-31VDC on the grids.
checked the other tube - it was pulling about 50mA.
switched them around to see if tubes or sockets and the readings moved with the tubes.
time for new tubes?

i then checked the power output with a 100mV sine wave into my (brand new) dummy load - it was giving me 12V p-p - so 4V RMS, so it's only making about 4 watts (into 4 ohms)?!?!?

this is a 50 watt amp...

only other thing that looked suspicious is the ripple on the main filter cap - about 8V p-p. is that acceptable? seems high....i thought the plate voltage seemed a bit low....

how am i doing?

(as far as i can tell the attached schematic corresponds to this amp...)

both heaters are running, as far as i can tell.

just two glowing points, one on either side of the tube, right?