(almost) first build advice needed - TW inspired

[flood]

hope this is the right section...

hi to all from bombay, india

i've been browsing around these forums for a while. after i recently had some success with an ultralinear EL84 hi-fi tube amplifier, i've decided to roll out another one, this time with an octal output section. the other project i'm working on is doug hammond's firefly.

this amp takes a bunch of features i've seen in a number of different amps. i wanted some advice from the gurus out here as to whether i can proced with the build with no restrictions, or i should consider changing things around.

not shown on the schematic: provisions for switching between a trainwreck-style and vox-style tonestack directly after the 100n cap. that is the reason i put the cap there - i don't know if it is a good idea to switch the HV coming off the paralleled plates.

not that interested in a gain monster, but mostly power tube saturation, a large range of tonal flexibility, killer blues to rock tones etc. lifted theoutput stage from the allen class act amp, becase i want it to take a number of tubes - 6l6, 6V6, EL34 etc. will the cathode biasing do (470R 15W)?

i have an old PT i salvaged from a tube radio, and will be using a custom wound OT with 5k primary for about 8W max output.

please write in with tips...

[flood]

...nobody?

thanks in advance.

[Blindog]

Hello flood, welcome!

Please don't take this the wrong way, but I think you may get more feedback on your project if you can come up with a schematic that is more clear and complete. There are components drawn with unresolved connections, and the general way it's drawn makes it kind of hard to read right away. It would be nice to see your power supply as well. The way it is designed can have an effect on the how the amp sounds. Just my opinion Good luck!

Mark

[Jana]

Yeah, the schematic is kind of hard to read. But if I am reading it right, I would put a 47k or so across the gain switch or it is going to pop like crazy when you throw that switch. You don't need the .1uf cap going to the tone stack, use caps in the tone stack rated for voltage instead. Why two caps in parallel going to the power tube? I would have a cap higher than 16v for the power tube bypass cap. Why all the 47pf caps on the input? do you live under a radio transmitter tower? LOL.

[Bob-I]

Looks pretty good, I'll bet it'll sound good.

On top of the other suggestions, I'd recomend another filter stage on the B+. Single ended amps tend to hum so after the 47u I'd put a 1.5K/5W and a second 47u.

Good luck.

[Blindog]

+1 on the additional filtering.

flood, when I enlarged your schematic it cleaned up some...my bad! It was late when I looked at it

Mark

[flood]

hey guys,

thanks a lot for the tips. just to clarify - do i need to resize the schematic? didn't think it would pose much of a problem, but i didn't click andtest it myself either if you think it'll help, i will be glad to redraw.

jana, you need to come to mumbai, india to understand what RFI is ! with a population of 17 million, and approximately as many mobile phones, airconditioners, fans, lousy AC mains etc, you have a fair amount of HF hash all over the place. the 47pF cap idea was something i learned while designing ECGs during engineering studies in germany. i was surprised by the efficiency of ths simple RC network, and thought that i would not miss the sligth HF cut...

the two caps going to the power tube are 1nF ceramic paralleled with 1 nF polyester or trop. fish. lifted off some express schems that i've seen (i believe it'S all over the famous wreckxpr.pdf....). must the two caps be equal in size, or can i use them "lopsided" to get the total capacitance value? like 470pF dipped multilayer ceramic // 1500 pF MKP?

i don't know if i'm right, but i believe that this contributes somewhat to a certain graininess or complexity inherent to the express. i'm just a noob, so please shoot slowly

i've decided to skip the switchable tone stacks and use the standard wreck TS. it wasn't the complexity, but the space constraints - i will probably want to revisit that idea some other time. i'm using a cakepan for this amp!

i will incorporate the additional filter - i seem to have overlooked that altogether. jana, a stupid question - how should the 47k resistor to defeat the pop on the gain switch be wired?

thanks to all for the feedback! i've gained a LOT of knowledge from this forum and the great attitude of the people on it, hope i can contribute some day!

[Jana]

I commented on the 47 pf caps with the joke about living under a radio transmitter because I lived in an apartment once that was only a few blocks from a radio transmitter. I too had to put a 47 pf cap across the input, I only used one though.

put a 47k to 100k resistor across the two terminals of the switch. when the switch is open the high resistance (in comparison to the rest of the circuit) will not change the effect of "unswitching" the cap. What it will do, however, is reference the cap DC voltage so that when you close the switch there isn't a sudden surge of DC voltage to charge the cap, which makes a pop.

[flood]

ahhhhhhh ok, i get it now. will do that. thanks for the heads-up! "enclosure" is almost drilled completely, can't wait to get cracking!

[flood]

progress report...

this build has gotten quite far already, pretty much the only thing that is remaining is the power supply, adding a bright switch and standby and wiring up the speaker jacks.

now this is what i am rather worried about:

i decided to give it a test run after completing the heater and power switch/fuse/IEC socket wiring, with all three tubes installed and also putting in all three power tubes i have in mind... even with the EL34 there was no problem whatsoever, and the tubes were all glowing beautifully. a bit too beautifully. i pulled V2 and measured 7.45 VAC across pins 4 and 9! same thing happened measuring from 2 to 7 on the octal socket.

i am confused - although it has been a long time, i am quite sure the tranny i am using is wired up correctly. i will try the other two leads just to make sure anyway. if i'm not wrong, the output of the transformer was about 200-210 VAC. that could explain the difference in voltage. or, the transformer was wired for 220V which was mains voltage back then in germany, we have mains closer to 230-235 in india.

will the extra volt be a tube killer? i believe the threshold is 7.2 V.


the next thing i am REALLY worried about - i pulled this tranny from an old siemens tube radio (working) at a flea market. it had about 5-6 tubes including an EZ81, which is why i thought it would be suitable for powering even EL34 filaments....

BUT: i didn't think about the current that B+ will require. IIRC, the power tube was an ECL82 - AFAIK there is a substantial and scary difference between the current needs of this tube and the 6CA7 or 6L6 i wanted to use. what worry me are the thickness (or lack thereof) of the wires on the tranny - very thin single strand insulaed copper.... do you more experienced people here think that it's up to the job?

third fear: tranny hums like a bitch and is located right next to the OT, albeit rotated 90 deg. in the vertical plane. am i correct in assuming will need to install a grounded shield of some sort separating the two?

i have taken pictures and will upload soon as i find my card reader. otherwise, i'm pretty satisfied till now... please write in with advice. all help is greatly appreciated.

[flood]

problem fixed - i had wired the mains transformer backwards i get 6.7VAC at the heaters now, and it seems to have more than enough current reserves to power both pre tubes and the EL34 heaters...

am thinking of a way to wire up the power supply without using a board (very little space...)

i still have a couple of things to finish - complete the power supply, install bright, triode/pentode and standby switching, and speaker jacks...

i will post pictures soon, i promise just don't balk when you see them, because this amp has possibly the grungiest, shoddiest exterior you'll ever have seen - it is my second build, and the basic principle behind it was to build the best amp i can from whatever parts i have lying around. also, i've never done a P2P build before. so do throw your brickbats... but please throw them gently

[Cygnus X1]

Can't wait to see it, flood...and to hear it!

I have focused not on appearance...that can come later.
It is the sounds I'm getting that keeps driving me to make more amps.

Whenever I get around to making them look better, I might be able to sell some for more building opportunities!

[flood]

hey all, almost done with this... one last question - i want to install a switch to go between pentode mode and triode mode. is it ok to install this directly between pins 3 and 4, or will i need to diconnect the 1k g2 resistor first and take the g2 voltage off before connecting to plate? thanks!

EDIT: ignoe that... i used this: http://www.tone-lizard.com/images/Pento ... switch.gif

[Structo]

Looks cool.

Welcome to the forum.

Sorry about all the troubles you guys had recently over there.
Prayers for the city.

[flood]

Looks cool.

Welcome to the forum.

Sorry about all the troubles you guys had recently over there.
Prayers for the city.

hey tom, thanks for your wishes. it was definitely a shocking incident, and it could pretty much have been me or any of my friends who could have been at the places that were attacked. i'm grateful that nobody i know was hurt or affected directly by the attacks.

sad part is that a lot of other tragedies, both terrorist attacks as well as avoidable problems, such as the deaths of thousands through drought or the agricultural crisis go unnoticed. the attacks shook up the "elite" strata of bombay, which is a good thing, since there is a marked increase in the elitariat and intelligentsia taking an interest in the accountability of politicians here.

i lived in germany for 7-8 years, and only moved back here 9 months ago. despite having lived here before, i find the levels of corruption and the general lack of discipline and civic sense mind-numbing. i suppose you either have to live with it, or fight a system that is being grudgingly (or not) accepted by a billion people.

...or move abroad (which the best-educated indians sadly do - can you say "brain drain"?).

anyways, don't want to ramble on - should have this thing up and running by monday. i'm going to take it easy and lay off the EL34 for a while. i don't know if the PT can supply the mA that would be needed...