YBA-1 Chronicles

Recently, through an extraordinary act of generosity I obtained a Traynor YBA-1 to add to my Traynor collection. The YBA-1 is a 2xEL34 50 watt two channel amp designed primarily as a bass amp. The model name assigned by Traynor is the Bass-Master, but lots of guitar players swear by them. Early YBA-1s (First-Gen) followed the Fender Tweed Bassman 5F6-A schematic pretty closely (despite the change from 5881 to 7027A power tubes). Later versions switched from 7027A to EL34, and therefor strongly resembled the Marshall JTM-45 (which was also borrowed from the Fender 5F6-A Bassman). Accordingly, many people look at the YBA-1 as the poor man's Marshall, since with a few minor mods it can be made almost identical to the JTM-45. On the other hand, there is a cadre of Traynor fans who say the YBA-1 is superior to (and more reliable than) the Marshall.

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.

First-Gen YBA-1

Some of the significant similarities among the First-Gen YBA-1 and 5F6-A include:

Significant differences include:

Interactive network input circuit.

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.

Cathode Follower with Interactive Tone Stack

One of the most significant elements of the circuit is the cathode follower tone stack. Rather than take the signal off the plate as in most circuits, the signal is instead taken off the cathode of a triode with no plate load resistor. Many guitarists feel that a cathode follower circuit gives a better "touch".

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".

Long-Tailed Pair Phase Inverter

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.

Rectifier and Power Supply

The 5F6-A is GZ34 tube rectified, with a center-tapped power tranny listed as 325-0-325. The 5F6-A has the stand-by switch immediately after the rectifier, followed by two 20uF/600V caps in parallel (40uf/600v equivalent), followed by a choke, and then multiple 20uF/450V caps at the screens, phase inverter, and gain stages. The schematic lists the voltages as 432V on the plates and 430 on the screens.

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.

47pF Cap Across the Phase Inverter

The 47pF cap across the phase inverter you might call the "anti-ice-pick" cap. Since the signal on either side of the phase inverter is 180 degrees out-of-phase, connecting the two signals cancels each other out. Connecting them with a very small cap means only the very high frequencies get canceled out, and the amp is smoother and less sharp. Given that the YBA-1 is primarily a bass amp those frequencies would be high harmonics and probably not missed. With a guitar plugged into the bright channel you might need this cap.

Power Amp Tubes

The 5F6-A and the YBA-1 have pretty similar power tube circuits, except that the 5F6-A specs 5881 tubes and the YBA-1 specs 7027A. 7027A tubes are more commonly associated with Ampeg amps, and not often seen in other brands. Both the 5881 and 7027A are derived from the 6L6, and share common pinouts. You can see a lively discussion about the merits of these two tube types on the internet, as well as some speculation about NOS versus current production tubes. At least at the time these amps were new, the 7027A handled higher B+ voltages and produced more watts. The bottle and plates were longer, and it used additional non-heater pins to conduct heat. The Traynor schematic does not show those pins wired, but for that matter neither does Ampeg, and maybe they were just jumpered on the socket. The Ampeg VT40 does show a frightening 586V B+ on the plates of the 7027As. The YBA-1 has no voltage indications anywhere on the schematic. The power tranny is part number 78632, but I haven't been able to trace that to anything. I would guess the YBA-1 ran at closer to 450V than 580V.

Second-Gen YBA-1

The Second-Gen YBA-1 has some significant departures from the First-Gen. Velvet Black gives the first year of the change as 1971, and the schematic available shows 1972. The most important changes are:

Significant similarities included

Gain Stage Cathode Circuit

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.

New Transfer Circuit

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.

Tone Stack

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.

Power Tube Conversion

The Second-Gen YBA-1 was converted from 7027A to 6CA7 power tubes. At the time of the conversion 7027A were not uncommon (as they are now), so I don't think the switch was due to limited availability of the 7027A. By this time Traynor was also making the YGL-3 (Twin Reverb style) with four 6CA7s, so perhaps they were consolidating their designs somewhat.

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.

Power Transformer Conversion

The First-Gen YBA-1 had a center-tapped power tranny with a full wave rectifier. The Second-Gen employed a non-center-tapped power tranny with a bridge rectifier. Since the amps were solid-state rectified, neither amp had a dedicated 5V circuit for a tube rectifier. On the First-Gen, the schematic shows the bias supply coming from the mains, whereas on the Second-Gen the power tranny had a separate bias winding. There are no voltages noted on the First Gen schematic, so it's not possible to know the difference in bias voltage in the change of power tubes and transformers. On The schematic for the Second-Gen I have available the bias is too fuzzy to read, but is -30 something.

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.

Choke

The First-Gen YBA-1 had a choke (unknown Henries) after the first set of filter caps and the stand-by switch. In this position it wouldn't influence the B+ voltage, but it would quiet and stabilize the B+ going to the phase inverter and gain stages. Chokes are generally absent on later Traynor designs, but other than an obvious savings in cost it's not clear why. It's noteworthy that the First-Gen had only a 10uF cap on the gain stages whereas the Second-Gen had 40uF but no choke.

Filter Cap Circuit

The filter cap circuitry of the Second-Gen was simplified some, and stiffened up as well.

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.

My Personal YBA-1

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).

Power Supply

Although the schematic shows a three-pronged power cord (still with a ground switch, however) my amp arrived with a relatively low-gauge (for a 3A device) two-prong power cord. Some elements of the power supply wiring before the transformer were done with lamp cord, as opposed to high voltage solid core wire, so it's likely the cord is not original. I suspect someone whose house lacked three-prong outlets got sick of using adapters and converted the three-prong cord to two-prong.

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.

Capacitors

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.

Test Drive

The amp was working when I got it as far as I know. I didn't have a decent speaker cab to test it with initially. So, I swapped in a new set of tubes (the 12AX7s in the amp were vintage Philips, and the 6CA7s were mis-matched Philips and Sylvania). I put an 8 ohm dummy load in the speaker jack, connected it to a current limiting device (two 60 watt incandescent bulbs in series to the power supply) and fired it up. I got some initial current rush as the filter caps charged up, and then a low glow at steady state. Sweet!

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.

What's It Sound Like

So far, I have played a 60's Epiphone Riviera (like a Gibson 335 with mini-humbuckers) and an early 60's Gibson SG with 490 humbuckers through it. I haven't tried it with any single coil guitars yet.

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!