Bombacaototal wrote: ↑Thu Mar 05, 2020 9:54 am
Thanks Donvan, very helpful.
I am on a bit of a scratching head situation. My amp has a 240V/230V/220V switch. The mains in central London are usually between 246V and 253V and therefore I have been running the amp at 240V. I like the option of been able to lower if travelling to a place were mains are running low, for example sub 220V. The mojotone supplementary transformer onle has 230V/220V taps. I was thinking in hardwiring it at 230V, but with the high mains in UK it will (i) lose around 250mA of current and (ii) increase the secondary voltage. In the first case there should be no problem as I will be using around 2.5A our of the 4A available, but the secondary is where the headache starts. I think I may need to drop secondary voltage if going straight from the mains in the UK (via resistors as per article linked by donvan), but then if going to mainland EU where mains are between 228V and 235V will get the secondary maybe too low? And what about a sub 220V scenario? Any suggestions on a work around this with the transformers I currently have?
The easier option would be to replace my bassman transformer by a twin transformer which will give me plenty of current, but there is the extra cost of getting a new transformer..and all the work in swapping everything....other option would be to find a supplementary filament transformer with 240V/230V/220V ...
It looks like current is not an issue for your filament transformer. As far as the filament voltage goes, you need to keep it between 5.7v and 6.9v. If you hard wire the 230 primary of this transformer to the mains, that should give you about 6.7v for the secondary voltage if the mains voltage is 246v and 5.9v if the mains voltage is 215v. Seems like that should work.
Thanks for sharing Marcus. Did you build it, or do you mind sharing the maker and model? My Eurovolt can drop the mains voltage, but still oscillates. It would be great having something like the kikusui PCR 2000M that keeps the voltage constant.
Bombacaototal wrote: ↑Fri Mar 06, 2020 9:37 am
Thanks for sharing Marcus. Did you build it, or do you mind sharing the maker and model? My Eurovolt can drop the mains voltage, but still oscillates. It would be great having something like the kikusui PCR 2000M that keeps the voltage constant.
It's just a standard APC UPS not sure what model number as I'm not at the studio (I think it's 1500 VA). It keeps stable filtered power for my computers and anything like that mainly that need 'good, clean' power. Also helps if there's a power cut
Not sure how useful it would be for your amps, as they're going to be plugged in all over the place when you use them live. I think a variac would be money better spent and cheaper too.
I think a buck/boost transformer in a box would be a good option to deal with voltages from 220- to 240+ for an amp (as in one unit) set up for nominal 230. This is another thing to carry around, but it would be lighter and cheaper than a Variac.
Back at this one as I am back studying this circuit. On the bias section, it has 2 bias pots. Should I expect that the 2 tail resistors (highlighted in yellow) to ground will be same as a "normal" bias structure, ie 27K for 6V6 and 6L6? Is 10K correct for 6550?
I am also thinking that the other 2 values are resistors in series to get about 77K.
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Nothing tricky about that circuit. Looks normal to me. That circuit will allow the pot to swing from -15v up to -30v on the pot wiper. If this voltage is applied to the grids, it's not nearly enough for 6550s. You probably need a range of about -45 to -60 for 6550s, depending on the plate voltage. Maybe there's more to this circuit than you are showing?
sluckey wrote: ↑Thu Apr 29, 2021 2:01 pm
Nothing tricky about that circuit. Looks normal to me. That circuit will allow the pot to swing from -15v up to -30v on the pot wiper. If this voltage is applied to the grids, it's not nearly enough for 6550s. You probably need a range of about -45 to -60 for 6550s, depending on the plate voltage. Maybe there's more to this circuit than you are showing?
OK, so the bias goes through a CF before it hits the 6550 grids. -30V into the grid of a CF is probably gonna give about -27v on the cathode and that's what will be seen on the 6550 grids. Not enough IMO. I think those 47K resistors are either wrong value of not connected in series as you show.
sluckey wrote: ↑Thu Apr 29, 2021 4:38 pm
OK, so the bias goes through a CF before it hits the 6550 grids. -30V into the grid of a CF is probably gonna give about -27v on the cathode and that's what will be seen on the 6550 grids. Not enough IMO. I think those 47K resistors are either wrong value of not connected in series as you show.
Whoever traced the circuit marked these 3 components (10k, 30k, 47k) as part of the bias circuit (attached). I am just trying to figure out what goes where..any guesses?
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sluckey wrote: ↑Thu Apr 29, 2021 6:21 pm
Wild ass guess... Put the 47K in parallel with the 30K. Now the pot wiper can swing from -39v to -78v. Now that's a believable range for 6550s.
Thanks Steve, interesting theory, surely worth trying when I build this. I had the impression that Taylor used 68k on his 005, hence why I thought 30k and 47k could be plausible, but I trust your math more
