New Tim Mellows OTL project

[smith01793]

just as a footnote = a picture of one of the overheated tubes

these 6c33 tubes really are rugged once burnt in properly,
despite this tube glowing cherry red when being over run (to the extent that the getter deposits on the inside of the glass are burnt) the tube still works ok and the bias sets up the same as a one of the other tubes when I swap them around.

I really trust Tim's design 100% and 9 months on the lesson learned is that the off sett pots do not need to be easily accessible. you could get away with having the bias pots hidden too but. the benefits are that as you crank up the bias the damn thing just sounds even better

[flatheadmurre]

I have the multiturn trimmers hidden under the topplate so the drilled hole guide the screwdriver.

One DVM atached to 2 panel jacks connected to a 0.1 resistor to check dissipation and a another DVM attached to speaker posts to check offset.

That´s all it takes to adjust my OTL.

And agree with your experience ones they settle they are very stable and takes lots of abuse.

[rudrud]

Hello!
I want to put instead C8-C11 one C = 2200-250V, as discussed in the N96. What resistance should be put in place of R23-R26?
Thank you!

Merry Christmas and Happy New Year!

Rudolf

[smith01793]

Hi,

Tims design uses seperate resistors to to keep the voltages balanced across the capacitors, if using just two capacitors 100k across each will be okto discharge them on switch off.


Happy new year,

Tony.

[wlowes]

Originally Posted by wlowes

I am building a pair of otl monoblocks based on another design. I have read this thread from the beginning and hope someone can advise me based on the experience gathered here. My power tubes need +170-0-170. Circuit calls for a very standard dual power supply using a 230vct with ct to ground, and a bridge rectifier to CRC filters. ...

FYI, built the power supply and it works perfectly. Per discussion here, no CT to ground and all is well.

[kambule]

Voltages marked on Fig 1 of feb2010 audioxpress is strange. While the HT is +-350150 and -150,
the voltage at the junction of v2 and v3 is marked -350v ??
at grid of v5 -200 ??
at grid of v4 -50 ??

Would someone share the measured values please.

[cnpope]

Originally Posted by kambule

Voltages marked on Fig 1 of feb2010 audioxpress is strange. While the HT is +-350150 and -150,
the voltage at the junction of v2 and v3 is marked -350v ??
at grid of v5 -200 ??
at grid of v4 -50 ??

Would someone share the measured values please.

The voltages are about right. The voltage at the cathodes of V2 and V3 should be just a few volts positive of their g1 grids. The voltage on the g1 grids is set by the voltage divider composed of R8 and R10 (and also R9 and R11), with the R8 far-end at +100V and the R10 far-end at -430V. With R8= 4.7M and R10= 1.0M, that would means g1 is at about -337V. So maybe that -350V indicated in the schematic is a bit off, but only by 15V or so.

As for the grids of V4 and V5, the biasing for 200mA or so of anode current in the 6C33C requires that the grid should be at about -50V relative to the cathode. Hence -50V for V4, since its cathode is at 0V, and -200V for V5, since its cathode is at -150V.

I measured on mine, and those figures are in the same ballpark as indicated in the schematic.

Chris

[kambule]

thank you