I don't know if I've ever posted this YBA-1 mod to the group or not, but anyway here goes ... the mod consists of an attempt to extract the most in terms of transient fidelity from the YBA-1 signal path without altering the gain structure or intended dynamics ... these changes are better tried on an already modified early YBA1, an original unit in pristine condition would be better left alone in many's opinion ...
Traynor (YBA-1) Bass-Master Mod to "Source Circuit"
'58/'59 Tweed Fender Bassman/Twin (5F-A) *PREAMP* values
[1a] decoupling input bias circuits, i.e., replace the "common-cathode" 820 Ohm/250 uF bias circuit of the input gain stages by separate networks - I like 1k5/125 uF and 1k5/0.68uF cicuits values but you can use the Marshall 1959 values 1k5/250uF (chan1: more gain, more bass) and 2.7k/0.68uF (chan2: less gain, less bass). Cap value varies according to taste, the larger cap will give you deeper bass response, a higher cathode resistance reduces gain - many amps including Traynors seem to sound "woofy" if a too large cap is used here ... This mod is not in line with the original 5F6 circuit but rather is a Transient Fidelity increasing mod (for which the Marshall 1957 and 1959's are known for) ... many people use the "half-circuit" analysis to justify using 1.5k resistors in a separate bias circuit to produce equivalent gain levels produced by the singlt 820 resistor - this turns out to be wrong ... because the tube gain is so low (u=100 in a 12ax7) the half circuit principle doesn't hold and in actual fact if you wanted to have near identical Tweed gain levels in the front end you'd have to use 750Ohm individually according to my Spice sims (pretty suprising to me too that it's less!) ... now because one might omit the resistive divider in a single channel implementation of the circuit, the reduced gain levels produced by using 1k5's might be welcome ... experimenting with different values and carefully testing is the only way to tell which work best with your playing needs and the rest of your signal chain (mainly pickups) ...
Note: "About Single Channeling It" -> Many players superstitiously like to keep the original functionality of the amp perfectly intact ... except in a "separated cathode front end" version maybe have a large value on one side and a lean on the other as described above. I prefer [1b] disabling the second channel altogether and having a single well-selected (100uF) cap as the only cathode bypass cap in the whole signal chain - the amp then becomes a pure single channel, maximized transient fidelity signal path ...this is noticeable at all playing levels ... a single channel implementation of the 5F6 topology is perhaps its only potential improvement, all else being perfect many would agree ...
[2] change 4Meg volume pot(s) to 1MegA for a less "spongy" attack ... 12ax7 tubes can be grid biased with a 10Meg resistor, this means that a 4Meg pot should start interacting with dynamic grid currents - i.e., the sensing port of a triode seeing such a high resistance path to ground will superimpose it's sourcing impact on the boundary waveform between it and the previous plate circuit ... in other words a high grid resistor in the form of a vol pot will interact with grid current reacting to signal fluctuations ... hence a gain stage that will dance around too much - it is a subtle effect but noticeably improved when replaced by correct value 1Meg pots ... as well, a 4Meg pots means loss of highs in that part of the circuit dues to interaction with pF-level tube parasitic capacitances ...
[3] change tone stack to 5F8/6-A values, i.e., 47k to 56k and 0.1uF to 0.02uF. This network impedance matches well with the spectral nature of many typical guitar pickups ... amongst other things it brings in the midrange dip and narrows the treble and bass peaks closer together ...
[4] I like the old presence control, i.e. the old"scratchy" style circuit with only the 4.7k Pot and cap on the wiper. Sometimes, as an add-on I like to use a dual 4.7k pot wired to a DPDT switch (to yield a 2.5k equivalent pot) in the presence circuit in order to alter loop gain of output stage feedback mixing at the flick of a switch ... this tends to favorise pentode distortion in the amp and also loosens the output stage a bit ...
[5] You might like reducing the 100k feedback return resistor to 47k, this tightens the output stage ... set to taste ... do NOT disconnect the Feedback loop in any class-AB output stage ever, if you do so the pahse driver will not work in a balnaced manner and your amp will work lopsided ... it may make the amp feel nice and loose at low playing levels but at high levels the amp will fart out ...
The power supply B+ filters caps [7] could use a few more Farads, 80/450 caps can become 100/450 and 40/450 caps become 50/450; as long as these values are not exceeded this won't harm the PT ... on the other hand the preamp filter caps I leave as they are or else the amp will start ghosting notes (if caps are lowered in value) or will feel too stiff (if caps are slightly increased even) ...
You can also experiment with the triode/pentode ratio when the amp is overdriving by altering the preamp voltage headroom. The values R21=4k7 and R32=10k can be changed to 1k, 2k2 repectively ... I use 2Watt resistors for this ... you will notice a cleaner and slightly brighter response from the amp ... when cranked the pentodes will fire up earlier and give the amp a truely warm and sexy feel ... somewhat like early Garnets ...
I hope this encourages other YBA-1 owners to explore the hidden tone available in this underated beast ... especially those who seek enhancements that don't alter the original set of dominant expressive characteristics surrounding the amp ... these are my ideas and opinions only, feel free to add or change things to suit your own taste and needs ...
Btw, in the single channel version of this mod the discarded gain stage can be used, in conjunction with a relay circuit, to give a 2 channel switching amp that produces a jaw dropping 70's overdrive and compression .... but this is an extreme mod, albeit entirely compatible with the hi-fi mod detailed here, but requiring some potential chassis drilling to accomodate Masters that routinely accompany such mods ...
the above steps can be considered extreme and are better kep for an already modded unit ... a classier mod I favour consists of only changing 4Meg vol pots to standard 1Meg and nothing else ... only the overly spongy aspect of the BassMaster that often makes players run for Marshalls is dealt with and does minimal cosmetic damage to the amp's wiring ... as well, the disengaging of the second channel (by lifting a wire going into the resistive circuit) is performed if an extreme in transient fidelity is to be experimented with ...
Note: it can be argued that the early BassMaster needs no tampering at all, and here it would be hard to disagree ... paired with the right speakers it is often considered a perfect amplifier as is ...