Tone Control Stack Frequency Response for Traynor YBA-1A Mk II

[nevetslab]

When I had my Traynor 1970-vintage YBA-1A Mk II Bassmaster Amp up and running last month, and plugged in the shop bass I have here for checking out amps I service, it brought back my original feelings about it's tonal range from the early 70's. It lacked tonal range. Cranking bass up to 10, I'm still wanting to twist the knob off. That's after re-capping the amp, stock values in the tone stack, though using Sprague 715P Series Metallized Polypropylene.

This morning, needing to write up a tutorial for a colleague of mine in using the Amber 3501a Audio Analyzer in cahoots with the Velleman PCSGU250 Scope/Generator/Bode Plotter system, I first set it up atop the Traynor to run some response plots of it's 'stock' Tone Stack. It showed pretty much the same as I saw when I loaded the tone circuit into National Instruments Multisim spice program a couple weeks ago. I knew after hearing this amp again last month that the stock tone circuits would be short lived, though I haven't made any firm decisions what I would use.

Here's the results. My 'baseline' plot on all 5 sets of curves was setting all four tone controls to mid-position, then run a min/max set for the Bass Control, the Treble Control, the LF Range Extender (Midrange) Control & the HF Range Extender (Presence) Control. Then, a set of curves setting Bass to Max CW/Treble to Min CCW, then Bass to Min CCW/Treble to Max CW. I see why it has always left me wanting more.



As you can see from the response plots, you're lucky to get 10dB adjustment range on the Bass control, without having to drop the Treble control for a bit more. The Treble Control has the greatest range of adjustment, though it too isn't very impressive. The tonal range of the Traynor YBA-1 (one version of it, at least) looked a lot more intriguing than this, though lots more parts. Might go the direction of the Ampeg B15R, as I do like the sounds I get out of those amps. Midrange & Presence control adjustment range just don't yield much. I know this amp is patterned after the '59 Fender Bassman Amp. Is it really this limited?

The test setup uses the Velleman PCSGU250 as the Closed-Loop Response Analyzer, it's Sweeping Generator's output connected to a HP 353A Output Transformer/Step Attenuator for Balanced Output, the output of the Traynor connected to an Amber 3501a Audio Analyzer, used as a balanced conditioning amp ahead of the Velleman Scope Input, output taken from the Distortion Analyzer BNC. Amber 3501a's BW was set to 80kHz (3-pole LPF), and loaded with 16 ohm resistive load. Output monitored on a Tektronix 7633 100MHz 4-trace scope (7A18A plug-ins).

[MarkusBass]

It would be nice if you showed the schematic of the tone stack with components values. Then everyone could see that the tone stack is quite unusual - it's partly based on Marshall and partly on Fender. It contains some unusual combination of components which put together do not work correctly. You have two options; either go "Fender way", or "Marshall way". I checked the Fender version only - I get 20 dB bass regulation and this is more than you need. Take a look at the attachment. In this version you have to change C2 to 500 pF, R15 to 100k, C6 to 47 nF, the Bass potentiometer to 250k, short the Middle pot. The changes are marked with red circles on the drawing.

If you want to check Marshall version, please note that C2 should be changed to 500 pF and C5 to 22 nF. R15 could be decreased to 33k but I haven't tested this version. It seems that they wanted to build the Marshall version but they forgot to change few components as required. You can compare the schematic with Marshall Plexi to see the differences.
Anyway, the Fender version works great - try it.

PS1: of course you can simulate each version in SPICE and see that Ferder version works, Marshall version works (most probably) and Traynor doesn't .

PS2: please note also that there is no cathode bypass capacitor on V2A valve. This also causes decreased bass response of the amp. For a guitar this is most probably OK but not for bass guitar.

Mark

[Mick Bailey]

My own amp is a slightly later 240v export model - '74 I think. Stock apart from the ground lift being removed (also still need to get round to a recap..). The one thing it has in spades is ample bass - a really plummy, rounded bottom end that works well with a 5-string. I also find that the controls don't have much range, but it sounds good with what little it offers and I'm not driven to change anything.

The low range expander on my amp trims the extreme low-end a little, and the high range expander adds a little glassiness to the sound - they kind of act as sub-bass and HF controls. There's not much incremental change, but noticeable at the extremes of rotation. Maybe there's been some slight changes over the years. I sometimes get Traynor amps where the schematic pasted inside doesn't exactly match the circuit.

[nevetslab]

Here's the schematic of the Rebuilt Traynor YBA-1A Mk II, as it is currently wired up

Traynor YBA-1A Mk II Amp Schematic-1.pdf

Interesting. Easy changes, apart from needing to replace the Bass Control with a 250k pot. Audio Taper, I presume? I haven't played with any of the circuit values yet in the Tone Stack. I was also pondering adding bypass cap around V2A's cathode resistor R12. I have seen that cap selected as a 680nF for guitar work, switch selectable. I'll give those changes a spin

Many thanks,

Steven

[MarkusBass]

Easy changes, apart from needing to replace the Bass Control with a 250k pot. Audio Taper, I presume?

Yes, audio tapper.

I was also pondering adding bypass cap around V2A's cathode resistor R12. I have seen that cap selected as a 680nF for guitar work, switch selectable. I'll give those changes a spin

For a bass guitar and 820 Ohms resistor I would use 10 or even 22 uF. You can connect it with 10k resistor to the ground and use a switch to short the resistor (as a BASS BOOST switch).
Here: The Valve Wizard -Cathode Follower you have some info on the cathode bypass capacitor (this is for 1.5k).
If you don't have 250k pot at hand, you can check first the tone stack by Marshall. It's because in this case you have to change only two capacitors and one resistor (and add cathode bypass capacitor). Then, if you are still not happy with the sound, you can switch to Fender.
Also the bright cap (510pF) is not the best value for this amp. I wouldn't use here anything more than 220pF - maybe even 150pF. 510pF adds unnecessary hiss to the sound.
It seems to me also that R18 and R19 (47k) have too low value. Shouldn't they be at least 220k? In many amps they are from 220k to 1M. 47k forms a low-pass filter and this may influence the frequency response of the amp. At least I'm afraid that this may be the case. You may test it with your equipment.

Mark

[nevetslab]

I pulled up the circuit in my simulator, and see changing C4 from 1nF to 510pF is a significant improvement. It, together with changing the slope resistor R15 work together. I think I'll stay with the 22nF for C6, at least for the initial listening. Strapping the Midrange control's wiper to end of pot is a simple change, and gives a more effective Midrange control. I checked the result of strapping a 330k across the wiper and working end of a 1M Audio pot for dropping the bass control to 250k. It does change the law of the taper a bit, but not greatly....it will do until I get a 250k Audio pot to replace it.

Adding bypass cap across R12 Cathode resistor does help...a slight change to the overall EQ curves, while raising the overall gain of the stage by about 4dB.

The second channel, while I currently have it active, and had changed Traynor's 1000pF HF boost around the pot to 510pF, I'll be making use of that half of the triode for an Insert Send CF driver, with the Return coming into a Master Volume at the input to the LTPI circuit.

I too thought the bootstrapped 47ks R18 & R19 looked a little low. They're increased to about 120k with the local feedback. I had already increased C7, which was 22nF up to 100nF

I probably won't get a chance to get it back onto the bench before next week, having client work taking up the time. I already have some other changes to look at in the driver/output stage to try.

Thanks,

Steven

[pdf64]

'please note also that there is no cathode bypass capacitor on V2A valve. This also causes decreased bass response of the amp'

I don't see why that should be the case, and have not noticed that an unbypassed cathode has a restricted bass response, compared to it being fully bypassed.
Bypassing does reduced the stage's output impedance, but as it's buffered from the load by a cathode follower, in this instance the affect of that may be more limited than might otherwise be the case.

Is just a perception thing, or something that can be measured?
Pete

[nevetslab]

Mark (MarkusBass) brought that up. I pondered it at one point when I was first going thru the amp, and saw it as a place where I could pick up a little more overall gain, but at present, it is still without a bypass cap. I've seen other mod's on the Traynor making use of that stage with a 680nF cathode bypass cap for a low-mid boost (285Hz corner). I saw a minor shift in the LF EQ curve using a 100uF bypass cap, but the main effect is raising the overall gain. The amp as it stands has plenty of gain.

[MarkusBass]

'please note also that there is no cathode bypass capacitor on V2A valve. This also causes decreased bass response of the amp'

I don't see why that should be the case, and have not noticed that an unbypassed cathode has a restricted bass response, compared to it being fully bypassed.
Bypassing does reduced the stage's output impedance, but as it's buffered from the load by a cathode follower, in this instance the affect of that may be more limited than might otherwise be the case.

Is just a perception thing, or something that can be measured?
Pete

You are right, this is my mistake. I incorrectly described something that you can see on the attached picture. This is simulation of this preamp with cathode bypass capacitor changing from 1nF to 20 uF with 1 uF step. Of course only the gain changes (about 4 dB). But once you start doing this, you can influence the frequency response of the preamp by increasing or decreasing the capacitor. With smaller capacitor there is less bass and with bigger one more. But of course not more than it was with unbypassed cathode. Sorry for confusion .

Mark

[MarkusBass]

Looking at the schematic of the amp I have still some doubts about it. I checked several Fender amps and in all cases R18/19 (47k) were 1 Meg. The input capacitor (C7) varies from 1nF to 20 nF but in most cases it was 20 nF. Then, R37 (47k) grid resistor was never used. I'm afraid that R37 with R18/19 (small value) and C7 (large value) may not work correctly. Have you simulated the power amp?

Mark

[TimmyP1955]

The problem I see is that the max lows output is at about 70Hz, and it starts to trail off from there. Couple this with a typical cabinet response that is somewhat similar, and I'm not surprised that it's wimpy. (But then again, I've not plotted any bass heads, so I could be full of malarky :-)

[nevetslab]

The Grid Stopper R37 I haven't seen used before on the LTPI's, although I haven't looked long and wide for that. You're right...the grid resistors are most typically 1M, or well above 47k. When they are 1M, then the small value of 1nF input cap for C7 works, as the HPF that results is still comparable.. And, that really set's the LF corner of the overall power amp.

I haven't simulated the amp yet. In looking back at different Traynor YBA-1 renditions, I see where they had, at one time, used 470k and no grid stopper on the input side, then later dropped those gird resistors to 47k and the grid stopper was introduced. The earlier ones, having the 470k's were still using 20nF for C7. That's a LF corner of 17Hz, not factoring in what the real input Z is from bootstrapping. 20nF was there when I began, but along the way, I liked what I heard when I increased it to 100nF. That, of course, lowered the LF -3dB corner, too low as I look at the numbers now.

One could ask if, in the case of the 47k grid resistors and the 20nF input cap C7, was that mainly for keeping the LF rolloff higher to better prevent OT core saturation? Onset of core saturation on this OT is 30Hz @ 50W/8 ohms. Low B on a 5-string bass is 30.9Hz. At present, optimum load matching for the 6550's is 11 ohms, where I get primary plate impedance of 5.1k C7 value, before I'm thru with this amp, C7 may go back down to where it was. I'll probably end up with a Hammond 1650R OT, though I don't have any data on where IT begins to saturate.

I'm also not totally locked into the existing LTPI circuit...though I'm now straying off the topic of the thread on the Tone Circuits. I've another thread delving into the issues of the Output Stage and my pursuit to yield a 75 to 85W 30-40Hz+ amplifier with KT-88's (or 6550's).

[nevetslab]

Revised Traynor YBA-1A Tone Control Stack Freq Respone Plots

Following MarkusBass' suggestions on revising the Tone Control Stack per the schematic he provided (looks like a Fender Twin Reverb ckt, less the 250pF HF cap), I made the changes to Traynor. For the time being, not having a 250k Audio Taper pot for the Bass Control, I slugged the 1M A pot with a 330k, as noted in my previous post.

This does yield more workable range on the Tone Control Stack.

I've updated the schematic accordingly.

Traynor YBA-1A Mk II Amp Schematic-2.pdf

I've also run a new set of Frequency Response curves showing the changes. I didn't bother running anything on the Presence Control, as that's not part of the 'stack'



I haven't had a chance to listen to this, but it looks like there's more latitude than I had before. I'll probably change to a different Presence Control circuit than what's attached to the Feedback network

[MarkusBass]

This version of the tone stack has one disadvantage; if you turn the pots fully counter clockwise, there is not signal on the output. To avoid this you can remove the link between C6 and the MID pot (upper pin).
What is wrong with the Presence control? I think that it is typical way of implementing the Presence control, used in countless amps (Fender, Marshall). One thing that I would change is the taper for the pot. Most amp manufacturers use linear pot (as you have of the schematic) but I check that in order to hear linear changes in presence frequency, you need to use audio pot. And some companies use audio pots as Presence control. Try both since you can easily plot the frequency response of the amp.

Mark

[nevetslab]

Adopting Ampeg B15R & Sound City Circuits to the YBA-1

After modeling the Ultra Low, Bass, Treble & MF EQ circuits from the Ampeg B-15R (and SVT-CL), I found the Ultra-Low circuit lent itself to a variable-Low Mid Notch Control to give more tonality range for the usual Low-Mid scoop we find in the traditional tone stack. I've always liked the response of the Ampeg B-15R & SVT-CL Bass & Treble controls (a passive Baxandall tone circuit), and only made a minor change to that circuit, limiting the min/max range of the Treble Control. After fiddling with a number of Presence control circuits, I borrowed & revised one from a Sound City SMF Tour Series Preamp that added more than just the usual 5dB presence boost.

I played with the Ampeg MF EQ circuit, but I've only liked it as a low-mid cut control in use, and there is a lot of loss in that circuit stage, so for the time being, I settled on the Sound City Presence circuit. Interesting circuit, as you can see from the schematic and the response curves.

I also decided to use a 12AU7 following the two gain stages, taking advantage of it's lower amplification factor and better drive capacity as a cathode follower at the back end of the preamp. I'll fine-tune all this in the build, of course.

I was looking for a resonance peaking circuit around 3-4kHz, but saw I could kinda get there by using the HF EQ and the Presence Control combined. I haven't yet built this circuit up, but it does look quite flexible in the simulation. I had intended to scrap the second input channel to provide the gain stages and a CF stage to drive the Insert Send/Return jacks ahead of the LTPI circuit.

I've included the revised preamp/tone control circuitry, along with some response curves from the simulator.

Revised Tone Control Circuit-2.pdf



Disregard the heater wiring in the schematic....that was just to run the tubes on the simulator.