The truth is, however, that the YBA-1 went through some pretty significant circuit modifications over the years, and ultimately ended up a unique amplifier different than either the Fender or the Marshall. I won't go into the whole history here (see Velvet Black for an extraordinary accounting of all things Traynor) but rather point out some of the significant changes from the 5F6-A circuit over the YBA-1's evolution.
Significant differences include:
The interactive network input circuit combines the inputs from both channels into single circuit. The schematic below shows the input circuit (ignoring the B+ power to the plate load resistors).
Of note, the volume control of an unused channel is in series with the grid leak resistors of the second stage, influencing the voltage seen at the grid of the second gain stage. Accordingly the two channels are not truly independent (beyond the shared resistor/bypass cap on the first gain stage cathodes). In the owner's manual for a different model (YBA-4) Traynor notes "... you may notice that while playing through one channel, the other channel's volume control affects the sound of the channel that you are using. This is a design feature and many interesting results may be obtained by experimenting with various settings."
Traynor changed several of the values from the 5F6-A (some say in order to avoid being sued), specifically changing the volume pots from 1M to 4M (which is really quite unusual), and changing the second gain stage grid leak resistors from 270K to 100K.
The two channels are made different by one channel featuring a 0.001 uF "bright" cap across the volume pot. The 5F6-A shows a 0.0001 cap, which is truly tiny.
The circuit is said to be "interactive" because adjusting any of the controls influences the voltages through the others. This arrangement was first seen, I believe, on the 5F6-A Bassman, and it's noteworthy perhaps that the 5E7 Bandmaster and the 5F4 Super shared the cathode follower circuit but with much simpler tone controls, and neither is nearly as famous as the Bassman.
As before, Traynor changed the values of some caps and some resistors in the network compared to the 5F6-A Bassman. In addition, Traynor labeled the Mid control the "Low Range Expander".
As we have seen before, the exact values of the caps and resistors varies between the 5F6-A and the YBA-1. Perhaps the most significant is that the YBA-1 has much less negative feedback than the 5F6-A with a 100K resistor in place of the 27K of the 5F6-A. This should lead to a livelier, looser amp, although the use of a solid state rectifier would have the opposite effect.
The YBA-1 has an interesting circuit labeled "high range extender" attached to the phase inverter ground. Typically this circuit is called "presence", but the YBA-1 circuit is slightly more complicated than the varistor and cap on the 5F6-A.
The YBA-1 has a solid-state full-wave rectifier (six PH204 diodes) on a center-tapped power tranny (no voltage listed). Immediately after the rectifier is a very odd set of filter caps (left side schematic).
The YBA-1 filter cap circuit appears unnecessarily complicated. I have not had the honor of looking inside a First-Gen YBA-1, so I am just speculating here. The design would seem to anticipate the "totem-pole" design used in Black Face Fenders (right side schematic). As 600V capacitors went out of production amp designers were forced to develop alternatives that were suited to the high B+ voltages of then-new amps. Putting caps in series doubles their effective voltage but halves their capacitance (actually 1 / (1/A + 1/B) for unequal caps A and B, similar to resistors in parallel). The 220K resistors are more common in such a circuit than the 100K used by Traynor, but obviously either works.
Traynor generally employed Mallory (Canada) can capacitors. If the best unit available was a 80uF/40uF/40uF at 450 V, then it makes sense to wire the two 40uF units in parallel to get 80uF/450V, and then wire that assembly in series with the 80uF cap to get a a 40uF/900V filter cap. The 100K bleed resistors help balance the load with imprecise caps (and drain the voltage when the amp is shut off).
The YBA-1 followed that totem pole with a noise suppression cap just ahead of the standby switch, a choke, followed by two 40uF/450V filter caps and a 10uF/450 filter cap. The first 40uF goes to the screens, the second to the phase inverter, and the 10uF to the gain stages. The schematic shows two 12AX7s, and that 10uF filter cap manages all four plates. 10uF seems like a minimal cap for that service, but that's what it shows.
In the First-Gen YBA-1 the cathodes of both the first gain stages for both channels were combined into a single resistor/bypass capacitor to ground (left side schematic). In the Second Gen YBA-1 the gain stages were separated, the bypass cap was eliminated, and the respective grounds were brought back to the input jacks rather than a central chassis ground.
Since in the First-Gen YBA-1 the cathode resistor served two tubes it's resistance was set for double current. In the Second-Gen YBA-1 since each resistor only saw current from one tube its resistance was approximately doubled, resulting in similar but slightly lower cathode voltage.
Removing the bypass cap from the cathode ground circuit would reduce the gain somewhat, and "darken" the amp's tone response. Looking ahead, it's noteworthy that the Second-Gen YBA-1 has no bypass caps on any of the cathodes. I'm not aware of any other amps wired this way.
A new circuit was inserted in between the second gain stage and third gain stage (where the second gain stage used to feed the cathode follower circuit). The circuit consists of three caps in series, with two of the caps bypassed by a 3.3M resistor, and with a 100K resistor to ground.
The first cap (0.047uF) is not very limiting, and would act like a resistor in the range of 60K to 10K on a bass guitar from low to high, and 40K to 5K on a guitar. The circuit gets pretty complicated after that, with a 3M3 resistor in parallel with the first 0.01uF cap (forming a fairly restrictive low-pass filter). My colleague Rob Maher at MSU simulated the circuit on PartSim. Given the range of frequencies of bass and guitar the circuit acts as a high pass shelf filter, increasing from 40Hz to about 1kHz and then going flat. The net affect would seem to be to attenuate low notes and keep all the higher-order harmonics intact.
By far the most important element of the tone stack is its move from the cathode to the plate. This change is thought by many to be fundamental to the touch sensitivity of the amp. Other slight changes in the values of the caps were also made, one increase and two decreases in uF.
Tube selection is another area where you can get a lively debate, but probably many people would agree that the 7027A was a louder, less colored tube, and that the 6CA7 (like the similar EL-34) has more personality or color.
Technically, the change over required re-wiring the power tube sockets. The 7027A is a beam tetrode, and can be wired up with only four connections (grid, plate, cathode, screen, ignoring the heaters) so that unused pins on the socket can be used as convenient locations to mount the grid-stopper resistor and the screen resistor. The 6CA7 is a pentode, with a separate pin for the suppressor grid in addition to the other four, and will short out in a socket wired for a tetrode. Interestingly, Traynor's approach to this differed among models. In the YGL-3 the suppressor grid was wired to the bias circuit, whereas in the Second-Gen YBA-1 it is tied to the cathode at ground.
The significance of this redesign is that that the power transformer could be wound for lower voltage (the bridge circuit produces 1.41 times the secondary, whereas the full-wave produces 0.71 times the secondary voltage) while requiring more current (the bridge converts 0.62 of the secondary current whereas the full-wave conduct 1.0). The bridge rectifier design doesn't require a center tap to be brought out on the secondary, and might be cheaper to manufacture or more reliable in high voltage amps. Many of Traynor's other amps have employed bridge rectifiers.
The filter caps were probably implemented ast two 40uF/40uF/450V cap cans, with the first one supporting the plates and screens of the 6CA7s, and the second one supporting the phase inverter and gain stages. Of note is the increase from 10uF to 40uF on the gain stage circuitry which should lead to a punchier amp.
Based on the serial number, and date codes on the circuit breaker, my amp appears to be a 1975, which is pretty late in the evolution of the YBA-1. It follows the Second-Gen schematic as far as I have examined. It does, however, have a few idiosyncrasies. Here's a shot (with the lid off).
The wiring of the power supply was non-conventional, with the On/Off and Standby switches wired with the opposite functions. So the first task was to convert it (back) to a three-wire cord with a solid ground. Unfortunately, some of the wires on the power transformer were pretty short, so for the time being I wired the no-longer-needed ground switch as the On/Off switch since it was closer to the tranny.
The power supply is classic late-model Traynor, with a solid-state bridge rectifier (and seemingly ridiculously small diodes). In what seems to be a Traynor trademark, the Standby switch simply connects the negative end of the diode bridge to ground. The positive end of the bridge is connected to the network of filter caps and dropping resistors to achieve the various voltages required by the circuits.
Traynors employed Mallory 40uf/450V double can capacitors as filter caps. These were generally pretty reliable beasts, but like all electrolytic caps they have a finite lifetime (see http://www.nmr.mgh.harvard.edu/~reese/electrolytics/ for an interesting discussion of caps). The first of two of the double can caps (the one that sees the highest voltage just off the diode bride) had been replaced by two 40uf/450V single electrolytics (CDE brand), but the second is still in service in this amp. At present it seems to be working OK.
More problematic, perhaps, the schematic shows the output of the bridge rectifier at 440V given a 115V AC current. That's really close to the 450V (+/- 20%) of the caps, and in modern times many AC wall circuits exceed 115 V. I put a voltage meter across the output of the rectifier and got a steady 450V. That's right at the limit of the caps with no cushion at all. Sprague makes a 40uf/500V cap in their Atom series, and that might provide some cushion, but they're a little pricy (about $20 apiece) and getting hard to find. Nonetheless, that looks like the future.
Except for the first capcan, all the caps in the amp appear to be original. Because this design has no bypass caps on the cathodes anywhere, the only other location for electrolytic caps is on the bias power supply. I haven't measured the bias voltage yet, but it appears the caps are working.
The coupling caps are all the original yellow (supposedly Mullard) caps. I haven't checked for DC leakage yet but none of them appears ruptured that I've noticed. There are also some (what I believe to be ceramic) caps in the signal chain. These have a pretty bad reputation and I might swap them out for silver/mica caps.
I had a YGL-3 I have been working on with dueling Hellatones in it (a Hellatone 30 (broken in Celestion G12H30s) and a Hellatone 60 (Celestion Vintage 30s)). The YGL-3 has a slanted open-back cab that doesn't really do justice to the YBA-1, but it's all I had handy. With speakers replacing the dummy load, it crackled and popped for a while, but then settled down to a reasonably quiet level. The transformers hum noticeably, but it doesn't come through the circuit.
Man, it's loud! I should have expected that. In the first channel, it's pretty bassy, and I had to push the treble up and the bass down to get a good guitar sound. It's probably great for a bass, but I haven't tried that yet. The two inputs in each channel are quite different. I'm used to the high gain/low gain paired inputs on my other amps, but it seems to make more difference on this amp, with the high gain input noticeably louder. The second channel has the 0.001 uF bright cap across the volume pot and it makes quite a difference. The second channel is noisier (due to less attenuation of higher frequencies probably as all the pre-amp tubes are new), but it does produce a nice guitar sound with less need for radical tone adjustments to get it right.
For 20 years my main gig amp was a factory stock Traynor YGL-3, but lately I have been using an Allen Old Flame, similar to a black face Super Reverb. The Old Flame is GZ34 rectified with 2 6L6s (I run 5881s in it). I had somewhat forgotten what a solid-state rectifier can do. The YBA-1 is not just loud, but the front end of the note comes punching right out. When you pick a note you better mean it!
The YBA-1 does not have a master volume (and I didn't have any ear plugs) so as far as I could push the amp it was very clean, and even the Gibson 490 humbuckers in the SG couldn't break the amp up at a volume I could live with. At almost 40 years old, this YBA-1 is loud and proud. I look forward to getting a decent speaker cab for it and seeing what it can really do!