Yamaha T-100 Tube Guitar Amp designed by Marianno Soldano

[nevetslab]

nevetslab:

Going back to my thread, I got a most valuable hint from G1 when my voltage readings seemed odd. His advice was as follows: "specify whether 'bright' switch should be engaged, does it make any difference?" And the answer is YES, it makes a difference. The Bright switch must be engaged in order to see the expected voltages on the preamp tubes.

The other thing is on the Yamaha Service Bulletin for the mod. I would skip that. Back then, I was working on a T50 and T100, one with, one without the mod. I did not hear the problem that was being reported. So I would skip that.

I looked closer at the PCB pattern of V4, and see it's stock...doesn't have the mod where the plates were separated, along with the cathodes. I'll eventually get there, but I'm still getting the chassis powered up. Maybe not so important to reform the electrolytics, but can't hurt. Approaching half-way there.

I was looking for other date codes, and found the CTS date code begin with '137', followed by the year and the week. So, seeing codes like 9043, 9113 and such. Date code on the Accutronics tank is 31 225 91, which I read as 31st week of 1991. My other Accutronics tanks have similar number patterns. So, no doubt amp was built late 1991 or after.

Seeing both the BLK and GRN wires of the Reverb Drive Xfmr Secondary connected to Pins 8 & 1 of power tube V10 makes me want to move them elsewhere, just so I can perhaps wire the power tubes up to accommodate EL34's....assuming this amp does work. That's mostly a function that would be for using it as a test bed for screening power tubes, but not scraping the amp as a functional unit.

I'll know more when I finally get some tubes installed and see what we've got.

Thanks,

[nevetslab]

Late yesterday I finally got the chassis fully powered up, spending 6 hours incrementally increasing the mains voltage with no tubes installed. I found two pairs of Sovtek 5881's, though one pair was half the plate current of the other pair (13mA pair vs 24mA pair), and installed those for initial test, figured close enough to drive a speaker. Set the output jack for 4 ohms to drive my Ampeg SVT115HL4 cabinet, and plugged my burst pink noise source into the preamp input jack.

Looking at the schematic, it led me to believe I could mix the Overdrive Channel with the Clean Channel where those preamp output circuits have their Master Volume wipers join thru mix resistor to feed the LTPI circuit. But, doesn't work that way. You get EITHER the Clean Ch or the Overdrive Channel.

Started with the Clean Channel. I was unimpressed with the tone circuits, though the Bright function was aggressive. I also liked the sound of the Reverb (Accutronics 8A68D1B). Third and fourth characters in the type number don't make sense. The stamped ink legends are clearly 6 & 8 for the 3rd & 4th characters. Whatever. Like the sound of it, at least.

The Overdrive channel also sounds downright ugly. Engaging it requires pulling the Gain knob out. The Pilot LED changes from GRN to RED. I found the Clean channel more to my liking, but overall, I could get far more output level with a Fender Deluxe Reverb than thru this Yamaha T100.

So, I moved on to checking out the Effects Send, to see if the preamp circuit of the Clean Ch was prematurely clipping. I had found one used 12AX7 that was clipping on the positive half, and yanked it out, which helped, but still, unimpressive sounding preamp circuit. That left the Effects Return circuit, which has a gain stage, a cathode follower stage to drive the tone circuits and the Master Volume. Pulling the O/D Gain pot out switches to the O/D tone & amp; master volume circuits, which are identical to the Clean Ch. No difference there. From there, it feeds the LTPI circuit & the power tubes. So, from a power tube test fixture standpoint, this has some possibilities.

I connected the amp to my Amber 3501a Audio Analyzer and dummy load to see with the existing bias setting what I had for output. I removed the pair of 5881 tubes that were only pulling 13mA ea and installed another pair of Sovtek 5881s (Fender GT marked 6L6GB), and they pulled 18mA...a bit closer). So while the load sharing still wasn't great, it did get me enough for rated power. I didn't measure distortion...there was crossover visible in the waverform, and no doubt could get a bit more with tweaking.

The bias circuit doesn't come from a dedicated winding on the Pwr Xfmr secondary. It's derived from the HT secondary, capacitor-coupled via 47nF/400V, 56k resistor load ahead of the half-wave rectifier, a 47uF/100V filter cap, a 56k resistor feeding a 25k CTS trim pot for the adjustable bias control. For a test fixture, I'd want the bias pot on the chassis. Perhaps a pair of them, so both upper and lower halves could be individually adjusted.

Never been that fond of this type bias circuit. Setting this chassis up for use with EL34's as well as 6L6GC/5881's will require lower negative bias voltage, so I'll have to look at that. Maybe going the route Orange took with their Rockerverb 100, they selected 1k/5W screen resistors for any of the power tubes. Only one bias pot, so you had to adjust it per tube type, so obviously they had enough range to span that for 6L6GC's and EL34's. Same Output xfmr, so it being optimized for 6L6GC's would no doubt be the case here on this Yamaha amp. I did see in the Orange schematic, they DID tie Pin 1 Suppressor Grid over to the Cathode pin 8. HT fuses per half and NO Cathode resistors. I was looking to install 1 ohm 3W Cathode Resistors with sense terminals. Though at present, I just plug in four Bias Probes, which is simple and doesn't require chassis mod's (for 6L6TGC's/5881's).

While I had this amp powered up on and off for over two hours, I did find the power transformer already getting uncomfortably warm (not unlike that of a Fender Twin Reverb if left on, Standby or Idle for a good while). No issues, just something I noticed. Yesterday, by the time I got the unloaded chassis up to 120VAC in Standby (keeping the HT supply voltages off of the preamp filters), the power xfmr excitation current was 380mA/22.4W @ 120VAC. I had run the DC voltages up to 400VDC, then switched to S/B to go the rest of the way on the main power supply caps, as unloaded, I would have exceeded 450VDC where the preamp filter caps were rated.

[nevetslab]

Yesterday, I pulled the Bias PCB out of the chassis to first replace the 470 ohm 1/2W Screen resistors with 1k/5W Metal Oxide parts. I had already moved the two 80uF/450V caps off the bottom side of the PCB onto the top...series connected 1st & 2nd stage filters. 150k ballast resistors across them. I looked to see what the Bias range was with the existing 56k series resistor to the 25k Pot, wired as a rehostat to ground. It gave me -59VDC to -48VDC. Then, to see what I'd need for a bias range of say -33VDC to whatever the upper range would be with a 25k pot, I placed my decade box across the 56k resistor, and dialed it to -33.5VDC. Then, set the 25k bias pot to the opposite end, yielding -48.5VDC I had 100k across the 56k resistor, which is 36k. So, that seems like a reasonable range for both EL34 tubes and 6L6/5881 tubes with those two ranges.

I then powered down, removed the power supply PCB, and then carefully began unsoldering all of the rear panel AC Mains parts until I had the panel empty above the four power tube sockets. From there, I began unsoldering all of the external wiring to the tube sockets, leaving the screen wires, the plate coupling jumpers of the two pairs of sockets, the cathode ground wires, then finally the poorly installed heater wiring and finally got it so I could re-wire the heaters with what they had with their pre-tinned insulated GRN and BRN wires, and twisted each pair from each tube socket, going from the corner socket down to the V10 socket, next to the Preamp PCB assy. That last socket had a production-formed twisted pair of that same wire, and I wired that into place. Then, added the Pin 8 to Pin 1 Jumpers for the EL84 tubes.



I also formed a small terminal board for the Primary of the Reverb transformer wires that had been soldered to Pin 8 & Pin 1 of V10 power tube socket, and then had the shielded cable from there to plug into the Accutronics Reverb tank. Shortened all that up, parked it at the corner of the Preamp PCB assy, and wired that up, running the ground wire over to the main Ground foil of that PCB assy.

This morning, having a fresh start with the chassis, I checked what my tool access limitations would be for using my Whitney XX Junior Chassis Punch, as I'd have to be punching from the inside of the chassis, dud to the 1/2" flange at the top of the rear panel. I made a paper template, one on the outside, then one a copy of that to go on the inside surface. Used a glue stick to mount the two templates, verified the registration, and then center-punched the hold centers. From there I loaded the Whitney punch with a 1/4" punch/die and proceeded ahead with all but the hole for the RED LED's.



I removed the two templates, then fetched my 1/2" Greenlee Chassis Punch, which uses a 1/4" thru-hole to drive t thru for cutting the mtg hole for the two HT fuses for the two pairs of power tubes. That made quick work of the mechanical revisions. Deburred the holes, then put the bottom panel of the chassis into place, taped up the rear panel with masking tape so I could then paint over the original Text in that panel space above the tube sockets. While the paint was drying, I typed up new text following their panel text on my P-Touch label maker. There was space below the Preamp tubes and the top edge of the panel surface.



Now I need to wait for the paint to dry, so I can wire up the cathode resistor terminal test points, along with the fuse and LED, borrowing the LED Indicator/Fuse circuit from Orange Rockerverb HT fuses, which if either pair has a tube failure, the LED lights up. Then, I will have to drill holes into the top of the chassis panel, underneath the Power Supply PCB assy for the two Bias Pots and Bias Test Points & Tube Type Select Switch so I'll have what I need for using this chassis as a Power Tube Test Fixture. It still keeps the amp fully functional, while getting utility out of it for the eventual digging thru all of my pulls from the past 12 years to see what I can cobble together in working pairs and sets of tubes.

Rockerverb 100W Power Board.pdf

Rockerverb100-Manual.pdf

[nevetslab]

I've completed the Parts Retrofit for the two Bias Circuits for selecting either 6L6/5881 power tubes or EL34 power tubes. I did find, after having put all the wiring to the Power Supply PCB back into place, that the lead length I cut/installed for the bias pots & select switch were too bloody short. For now, I'll live with that. Getting the AC Mains wiring off the rear panel sure helped getting everything else nice and tidy. I put those parts back into place. During all of that re-wiring, I did have a sketched-out schematic to follow, just to keep me alert to the changes.

So, while I haven't yet plugged it in to see if I did any major screw-ups, here's the results:





Packing it in for the day. I'll find out in the morning if everything works as expected.

[nevetslab]

This morning's initial power-up found everything but the Bias Voltage appearing normal. No Bias at all, so had to turn it over, discharged the supplies, then unsoldered the RED and YEL wires between the PS PCB and the Preamp PCB so I could then lift the PS board up as far as I could get it. I didn't see anything wrong with the wiring, apart from having cut the four wires for the two pots too short. I removed those and installed longer wires so I could then tilt the PS Board up fully for service access. After having verified all the connections, I then spotted the problem. The wire from the toggle switch wiper got crushed between the PCB and the stud-standoff when I had closed the board down on it! Shorted out the bias voltage. That would do it. Thankfully no harm was done. At least got longer wires in place. Closed up the board, reattached the two wires I had unsoldered, then set it down to try it out.

I found a set of EL34 tubes, that looked to have come from a Marshall JCM 2000. They were marked 'microphonic'. I plugged those in.



The Bias Voltage range for EL34's is -51V to -35V, and for 6L6's, -61.5V to -47V. The set of EL34's I biased up had read out as 33mA, 38mA & 40mA, 35mA. Not a real tight match, but might still be usable. Tapping on the tube glass, with the output of the amp connected to my analyzer/scope did reveal minor response. I haven't yet listened to them, as this is just initial inspection. At least this is a working Amplifier, so I can listen to what I have plugged in. It does look like it will suit my purpose. If I wanted to get fancy, I do have a 4-channel scanning HP 3467A Multimeter that could be connected to read the cathode currents of the four tubes. More trouble than it's worth, but at least I have part of the solution achieved.

Preamp tube fixture is next. The Driver tube in this amp, while being a 12AX7, could easily be substituted with 12AT7 (LTPI) for checking those out. I'll have to give some thought on what I need and what's on hand to cobble this next fixture.

[Leo_Gnardo]

If I wanted to get fancy, I do have a 4-channel scanning HP 3467A Multimeter that could be connected to read the cathode currents of the four tubes. .

Nevets, your test equipment collection continually amazes me.

[nevetslab]

I think now I need to finally get my Hickock 539A Tube Tester restored, as while this power tube test fixture/Guitar Amp works well for batching working tube together, marking the tubes that I'll be going thru to know what their transconductance is at a given plate current still remains. I haven't given thought on how I might do that with this fixture, though no doubt it can't be all that difficult. Yesterday, going thru numerous 6L6's of various types along with 5881's, I mostly found those that worked, but their plate current for a given bias voltage yielded vast differences. I'd find a couple pairs that were within 3-5 mA of each other, most were 10-15mA difference.

Nice to finally find a use for that Yamaha amp that I'd seen sitting up on the shelves become useful. First time I removed the bottom cover of that amp and see the rats nest of wiring that was just unskillfully cobbled together nearly scared me away.