SUMMARY FIRST DUE TO MY STUPIDITY – I COULD HAVE HAD IT GOING 2 DAYS BACK WITH BETTER REASEARCH ON 6AX7 heaters to realise why triode 1 was not working and NOT assuming the original valve was dead when tested in a normal amp – it was fine! Dick…
What Modder’s need to know that I found out the hard way over 3 days.
1: 6AX7s and 12AX7s are NOT interchangeable in this ODT100 model because the heaters are wired differently, so the first triode will not heat so won’t switch on, so cannot feed triode 2 as below – having 0V at both the grid AND cathode of triode 1 had me head scratching for ages knowing it could not amplify with no voltage difference between grid and cathode, when all other key components tested OK! The triode was OFF due to no heat:
Circuit diagram (some incorrect component PCB numbers) from:
2: The best options for altering gain or tone is to change only the resistors and/or add a bypass cap for triode 1 ONLY so you don’t affect the output impedance of triode 2 feeding the tone stack after C4 above. Triode 2 is the distortion stage.
Also, triode 2 has classic “clean” gain resistor values (100k/1.5k) to faithfully reproduce the distortion produced at triode 1 – however – there is 10Vpp being fed to the grid of triode 1 for a 100mV input (100 gain from the op amp up full), so triode 2 may be being pushed hard and overdriven too, in the un-modded version). Nothing to stop you trying it though…
These triode 1 mod options are putting other values for R3 (150K) or putting another resistor in parallel with R25, the 10K, and/or a bypass capacitor for local NFB.
R25 is a micro resistor on the underside of a mini PCB so cannot be changed directly. By tracing the jumper pins from the valve, and finding a ground at another resistor on the PCB, you can put a suitable value in parallel with this 10K as below– for example another 10K to give a cathode resistor value of 5K as suggested in that forum. This should give less distortion and more gain at triode 1 so seems pointless to me cleaning it up more, for a distortion pedal, you would just be overdriving the grid more at triode 2 instead – may get grid limiting and a form of compression – draw the load lines and work it out logically, don’t just use trial and error without some thought. I haven’t drawn load lines myself so don’t take my word for that last statement.
Er…more thought. It is a bit more complex than usual too with such a large cathode load – I measured 9V here – so a bias of -9V relative to the grid – this means the triode does not even conduct for the first 6V of that 10Vpp signal I mentioned – this means only about the extreme halves of a full sine each side is being amplified. Read Blencowe’s section on drawing the Cathode Load Line here to understand why it would be off the graph at -9V:
The solder points I found for this R25 cathode load jumper are seen below:
Check these points to ground with your DDM yourself, with the unit OFF and discharged!!
The anode load resistors can be de-soldered quite easily and replaced:
I ended this repair with a T1 anode load resistor of 82k when experimenting to see why the triode was switched off in case the resistor was going open circuit under load, before I realised about the heaters. This value gives nice enough varied distortion, but I will change to the original so I know what the unit should sound like first.
I also have to solve a mains hum issue due to the changed transformer probably, by checking grounding of the circuit to the metal chassis (it was fine – so probably the crap transformer I used). The bass control doesn’t work either – bad contacts – need a re-solder also. These pots are cheap too.
So – what a farce overall – but I learned the circuit due to the hassle and being stubborn. It’s the only trait I have that counters my overall stupidity sometimes…and I got it going even though it now needs tweaks to make it useable long term – the mains noise is bad, maybe due to the non original transformer and AC PSU. It may go in the bin yet…so what happened to me get into and back out of this mess…? Read on, or not…
I took a very impulsive risk on one of these pedals as I wanted to know how these valve loaded pre amp pedals work as far as supplying a suitably high voltage from a 6V AC PSU, for a 6AX7 to run at its bare minimum HT. I have seen a Blackstar version – maybe a DS2 or similar – in a box at DBS and heard it cost 150 odd quid so wanted to know what that price tag justifies.
Seems Biyang can do similar for a hell of a lot cheaper – with other FX units too.
It turns out the principle is exactly the same as any other valve amp so they should be treated with the same respect from a safety point of view, especially as there seems to be a lot of people into modifying these pedals for improved tonal and gain reasons. The secondary is stepped up to about 240V AC to give possibly 300V DC or so, so be careful.
This was a spare repair but I did REALLY badly price wise as they are less than 50 quid new anyway (impatient – didn’t check the new prices first –idiot!), and this thing is really trashed – normally it would have been a bin job – but I am very stubborn and persistent at times.
They do sound interesting too which encouraged me to try and fix it, with a wide range of tones and good for raw metal players it seems:
FIRST BIG INCORRECT ASSUMPTION AFTER BAD RESEARCH THAT 6 or 12AX7s ARE INTERCHANGEABLE –DEPENDS ON CIRCUIT HEATER DESIGN/WIRING.
I checked the 6AX7 valve in another amp – duff (so I thought!) – so changed it to a spare 12AX7. I had a spare 9V AC PSU to use for this thing already. I thought I would check the valve anode voltage first with the valve out. I got some weird changing voltages and the PSU got hot also.
The stomp switch was stated duff in the Ebay listing, so I changed that also (mistake – it was fine and the right one – a NONE latching type IS required). I tried with the new (wrong 12AX7) valve in then realised I was in trouble when the LED didn’t light. Seems like a short somewhere.
If it’s the logic components I’m in trouble. 3 chips and 6 transistors – not good.
I checked most with the scope component tester, and they seemed OK, and the LED and rectifier diodes.
The 6V transformer windings were very low – 0.4 ohms so I had to remove the transformer to be sure of the low reading – the same out of circuit. Most probably someone has used the wrong PSU with this unit after losing the original and melted the primary short – this would happen if a DC PSU was used instead of an AC PSU – remember transformer reactance on my other Posts.
My first problem was replacing the transformer – the serial number did not exist on google, but I had a bit of luck realising that such a cheap pedal MUST have stock parts from somewhere – not ordered and made specially – and was probably a normal 240V to 6V charger type transformer used in reverse, so I opened up an old Black and Decker 9V DC, 240V AC PSU I had already messed with for guitar FX pedals.
The transformer was almost the exact same size and the pins were even in line with the holes in the PCB! Some luck.
When comparing the two, the 240V windings were almost the same resistance around 1500 ohms – OK, that’s a good sign too. I removed the rectifier diodes attached to PCB and the lead from it, leaving just the transformer.
The 6V/9V windings readings confirmed the old one was short – should have been more like 6 ohms, not 0.6 ohms:
Once I soldered the 9V PT back in, I got much higher voltages – 183V AC, and 236V rectified DC, and the LED now lit too, but I had to click the new (wrong) latching switch twice each time for it to change – that’s not right. When touching the switch connection pins with a screwdriver I found it’s a short duration connection like a tap tempo delay switch required here to switch the FX in/out function.
Above – new transformer AC secondary 183V
Above – new transformer DC secondary 236V
Above – Normal power to the unit again.
The valve glowed bright also. All was looking OK so far, so signal test time. This is where it got bad again…time consuming.
I set up for FLS with a 100mV input, with the output feeding into the TC60 so I could hear the signal, as this thing has true bypass when off, which I heard. When switching with the LED on, the signal got cut completely – not even noise coming out. This could be bad, if things haven’t been disastrous enough already – if it’s the logic components. It been 2 days to get this far.
I found a schematic for the audio chain from one of those Modder guys in the forum link above, which shows the first 2 amps are in one chip which preamp for the valve. I was pleased to find 10Vpp – a gain of 100 at the gain pot:
This feeds the first triode of the valve, and I got this signal at the grid too, but a really tiny amplification at the anode. I checked the load resistors were the same value as the schematic and connected OK, which they are – a 150K at triode1 and a 100K at triode2. This was my first clue to a reason but I missed it – went down the wrong logic path assuming that as the transformer had blown then component damage had probably been done too – not so. This meant a lot of time spent finding and checking components and paths unnecessarily.
Above: 100k Anode 2 output.
This mini PCB stage links to the valve base and is where the Modders like to increase gain at triode 1 by lowering the 10k cathode resistor to 5k, by soldering another 10k in parallel with it or changing these anode load resistors.
The mini 4 pin chip is the dual 4558C op amp that feeds the 0-100 gain to the grid of triode1:
After I finally realised the heater of triode 1 was the only possible cause left for triode 1 to not amplify, I realised the major mistake I’d made, so put the original 6AX7 back in. That got this thing going to measure some signals. The min and max outputs from triode 2 to the tone stack are about 20Vpp to 110Vpp:
The tone stack reduces this back to a 1Vpp max output to feed a normal amp input:
I’ll try to solve the mains hum issue and bass control before videoing it with the TC60 this Thursday at DBS.
I got some nice tones out of it before it started going mushy so it’s probably packing up, as it seemed doomed from the start, but at least I know enough to buy a working unit second hand cheap if I see one – they are good value for a distortion pedal at 50 quid new – it is a valve amp after all! It is ideal for people who only have a clean practice amp who want to experiment with real valve tones cheaply, and if you want nice and varied overdrive tones you have them. You can mod the unit quite easily too if you are a bit techie.
I am thinking of buying another unit so I know what it SHOULD sound like – but from a practical side it may be better to get the next models up that use 12AX7s so that spare valves can be sourced more easily and different makes tried also for tonal options.
On the whole I salvaged a bad situation for my money due to the education side of this hobby – even if this particular pedal never quite works right again. I know what is involved now with these units.
Persistence pays off.
Added 2 sound files of the Biyang returned to non modded state and with a 3.6k cathode load:
Ok – corrections for values. It was a 5.6k in parallel with the 10k micro cathode resistor at triode 1 giving 3.6k total.
Seems to have a bit more bite but mains noisy but yours may not be with the original PSU and transformer…personally I don’t think the tonal changes are worth the trouble and potential damage to anything. Companies don’t employ circuit designers for nothing…