Power Transformer for Sano B18W

[eurekaiv]

For the second time in a row, an amp I've been tinkering on (Sano B18W) appears to have a bad power transformer. There no specs on the schematic unfortunately but it seems like around 600V is typical for an 18W? If I'm correct, it'll need just over 2A for the 2 EL84s and 2 12AX7s so was looking at the following Hammond from AES that's priced right and fits the current mounting holes. Seem appropriate? I can't find anything specific searching for an 18W power transformer that give part numbers and I need an upright mount.

P-T272X - 620V C.T. @ 80ma. - 5V @ 2A - 6.3V @ 2.4A

I told a friend I'd help him get the old amp buried in his garage for 20 years working, which then morphed into "can I gig with this? can you make it reliable?" Not sure I'm qualified for that sorta thing so if anyone has any suggestions for this please let me know. I already recapped it and I've verified it comes up fine with a random power transformer I had in the bins (much too small though and only useful to troubleshoot unfortunately) so that's the last thing that needs to be fixed to at least get it working.

Also, before I started this I was comparing with the schematic on the site and there are a bunch of differences in this amp (the power supply and trem circuit are kinda different) and so I drew up the changes. Is there a way to submit an additional version to the site for future use?

[HotBluePlates]

... Sano B18W) ... bad power transformer. There no specs on the schematic unfortunately but it seems like around 600V is typical for an 18W? ...

P-T272X - 620V C.T. @ 80ma. - 5V @ 2A - 6.3V @ 2.4A

... a friend ... "can I gig with this? can you make it reliable?" ...

Will a Hammond 290PX fit?

305v x 1.414 = 431vdc, but the 5Y3 rectifier should bring it down.  If it doesn't (enough) I'd add series resistance between the B+ secondary wires & 5Y3 Pins 4 & 6.

If the uncertainty isn't good, I'd shoot for something around 250v-0-250v (cause 300v / 1.414 = ~212v).  You're not gonna lose much volume, and no one ever complained they were getting too much tube life cause their B+ voltage was not excessive.  Seems a good move for "reliability."

[sluckey]

I think 80mA may be too lightweight.

I used the Hammond 270FX (550 CT @ 150ma) for my Vox AC-15. The actual measured voltage was 302-0-302. It produced 350VDC. The entire amp required 100mA. The two EL84s draw 92mA. More info and schematic here...

     http://sluckeyamps.com/VAC15/ac15.htm

[eurekaiv]

The 290PX is way too big and it's laydown. The amp has an upright PT with 2.125 x 2" mounting which is basically the same as the 270FX. I figured the 550ct would be too low so I skipped right past it. I'll go for one of those. Thanks again for the help!

[PRR]

.....I was comparing with the schematic on the site and there are a bunch of differences in this amp.... a way to submit an additional version to the site for future use?

Whoa, what happened to Sano_b18w.pdf on this site-- 13 MeggleBites for a lousy 5-toobe amp???

Here it is in 50kB. Also a GIF you can mark-up in PAINT if you like.

You can upload reasonable size (not 13MB!) files as message attachments. You have to go into Preview. Since this occasionally fails, save your typing first (Ctrl-A, Ctrl-C). If it is noteworthy, holler, and Doug may put it in the library.

[thetragichero]

i traded an acoustic 370 and hartke 4x10 for a sano 20w and it's an absolutely lovely little amp. whoever recapped it missed a solder joint in the trem circuit so it rattled until i touched it up but i used it a number of times at church with a hollowbody jazzbox when we were doing more acoustic outdoor services and it worked great. such wonderful point-to-point with 90 degree angle component leads built by lovely ladies in irvington, nj

anyway, point of all that is it should work great for him once you get it sorted!

[eurekaiv]

Here's my updated drawing...

The red items are values/parts that were changed/added but that I'm almost certain are original. The blue items are things that were different that I'm almost certain were not original. The can cap had been replaced at some point with an 80/80/20 and the resistors went with it and the 220pf cap on the plate of V1A is missing and I'm pretty sure that resistor was replaced since it matches none of the others.

I didn't color the wiring differences in this amp but they are as follows: the last power supply node provides power to the entire preamp including the phase inverter, the second node powers only the 1/2 12AX7 trem circuit and is switched through the pot vs. off the footswitch jack. Also, the cathode bypass cap in the trem circuit was drawn as part of the switching circuit in the original schematic.

[PRR]

It should not be 5Y3 on a 6V winding shared with the audio heaters. Do we know what is really there?

12k B+ droppers does seem appropriate. Oversize B+ caps may make little difference in a single-ended amp, except stray buzz.

Yes, killing the LFO with just the cap or the R-C network is a choice which may be a running change or about part parasitics. Likewise a stray pFd cap here or there.

[eurekaiv]

It does have a 5Y3 and the appropriate filament winding. I just updated the new schem to reflect this. It's not single ended though, and 80uf seems extremely high for the first node on a 5Y3 and only for the trem circuit, which is how it was wired when I got it.

[2deaf]

The 25uF cathode bypass capacitor on the first half of the 12AU7 will kill the NFB.

[eurekaiv]

I assume that cap was from the factory. The original cap had a resistor wrapped around the legs (P2P and same as V1) and the cap and resistor are the same as most the others in the amp, they also look like none I've ever seen elsewhere. But point taken, I'll definitely try the amp without it once I get the new transformer in.

[PRR]

> It's not single ended

Duh! Coffee shortage here.

> 80uf seems extremely high for the first node on a 5Y3

Yes. If it is a high resistance PT, it could be OK. But you are replacing the PT?

> The 25uF cathode bypass capacitor on the first half of the 12AU7 will kill the NFB.

There's not a heap of NFB here. 12AU7 gain is 16, final stage gain to 16 r maybe 1.8, so forward gain of 30, reverse loss 47k:2.2k or 22, so couple-dB NFB. Enough to dam bass resonance. (Switching to 12AX7 won't work without re-biasing.)

[shooter]

Duh! Coffee shortage here.

I bought myself 10 jars for Christmas.......just in case