I also notice that it has less gain than my other builds when 'on rotation', but the sound makes up for it. It was originally a design for ECL86, and then EL84, so I suppose the EL34 could be a slight compromise.
Engineer Baby Huey with EL34 is used with Klipsch Belle clones and volume is plenty enough, sensitivity is 103dB for these speakers which creates a problem with hum for Engineer version of Baby Huey. I am thinking to tinker with this Bandolo version and maybe I get the hum under better control, one problem with Engineer Baby Huey is the heaters are feed by AC which i understand can be a source for hum. This Bandolo version I plan to use DC for heaters, fingers crossed it will get rid of the hum.
Some of the things I've changed on the original BHEL34 schematic to increase gain, are:
use higher B+, higher load on the 12AX7, lower value pot on cathodes of the 12AX7.
What is the expected power from your b+ and tubes config? What is the measured input sens, or input voltage required to reach clipping?
If I remember right, your pictures showed hashimotos OPT that are rated for el84 PP amp. I think I use the same for my Tubelab SPP, so realistically no more than 12-15w output.
If everything is works right, There is nothing wrong with listening at 80% of the volume.
There are too many parameters missing, first of all the used schematic with voltages.
I could be simply a lack of sensitivity, or the fact that the powerdrive cannot reach AB2, or many other facts.
I could be simply a lack of sensitivity, or the fact that the powerdrive cannot reach AB2, or many other facts.
Hi everyone, wanted to ask for a bit of advice on completing the boards. The set I purchased were mostly populated save for the tube sockets. The big capacitors were mounted on the underside of the board.
My concern with that is adjusting the boards later. It would not be accessible from underside of board, I guess small holes would need to be drilled in top cover to accommodate, but don't know if this is practical. I'm thinking to move the big capacitors to top of boards, solder tube sockets on underside. Mount the boards upside down from top cover - tubes on top all other parts on bottom. The transformers for hv and 50v ( two separate) as well as ac-dc heater supplies I think would be mounted on bottom plate. If need to adjust, unscrew bottom plate, tilt back top of amp, adjust. I don't have a chassis yet picked out. Any tips would be appreciated.
My concern with that is adjusting the boards later. It would not be accessible from underside of board, I guess small holes would need to be drilled in top cover to accommodate, but don't know if this is practical. I'm thinking to move the big capacitors to top of boards, solder tube sockets on underside. Mount the boards upside down from top cover - tubes on top all other parts on bottom. The transformers for hv and 50v ( two separate) as well as ac-dc heater supplies I think would be mounted on bottom plate. If need to adjust, unscrew bottom plate, tilt back top of amp, adjust. I don't have a chassis yet picked out. Any tips would be appreciated.
For those who have replaced R17 with a LED and used EL34s (planning on HV secondary of PT to be 330V AC) what did you land on for R16?
I think I will have to socket that position
I think I will have to socket that position
It depends on the negative voltage you have for the bias. The current through the leds must be around 2 mA.
Hi Everyone,
Making some progress on this project. I'm having some trouble getting bias down low enough. V2 & V3 on both amp board hovering in range of 600-700mv. R27 reading 10.6ohms, though I know multimeters are less accurate with lower value resistors such as this. Adjusting trimpots will not get values any lower, I would like to get bias down to ~500-550mv. Using El34 tubes. Any tips on troubleshooting would be most welcome 🙂
Also not successful with the next step, adjusting P5 to get 0V between "inner" leads of C1 & C2, I'm getting ~130V dc.
377VDC across C27, 58V ac at bias input. 6.25v dc at heaters.
Making some progress on this project. I'm having some trouble getting bias down low enough. V2 & V3 on both amp board hovering in range of 600-700mv. R27 reading 10.6ohms, though I know multimeters are less accurate with lower value resistors such as this. Adjusting trimpots will not get values any lower, I would like to get bias down to ~500-550mv. Using El34 tubes. Any tips on troubleshooting would be most welcome 🙂
Also not successful with the next step, adjusting P5 to get 0V between "inner" leads of C1 & C2, I'm getting ~130V dc.
377VDC across C27, 58V ac at bias input. 6.25v dc at heaters.
With 370 Vdc on the plates of the EL34 you are fine around 53 mA, so 530 mV across cathode resistors.
Does it change depending on the point you touch the pcb? If so, consider it 10 Ohm in your calculations.
You need to lower the value of the resistor between the diode and the bias trimpot, to have a more negative range available. What voltages do you have now on the negative nodes of the bias?
Set P5 in the middle and report the voltages on both sides of each capacitor.
Thanks for your guidance, Zintolo. Please see below for my findings.
Would this be the measurement between ground (tp3) and C12? The measurement I got for this is -22V.
C1 - 123V
Yes, between 10.3 and 10.7 ohms.
Would this be the measurement between ground (tp3) and C12? The measurement I got for this is -22V.
C2 - 2.5V
C1 - 123V
I mean voltages across C10 and across C12. I guess, before having the info, that with R39 at 10k tou’ll be able to correctly bias the amp.
Check the voltages on plates of V3, R10 both sides and R8 both sides. There’s something wrong there.
Check the voltages on plates of V3, R10 both sides and R8 both sides. There’s something wrong there.
C10 - 51V
C12 - 22.9V
R10 - 61V
R8 - 335V
R39 - has 33k resistors installed. I see BOM has this as 18k.. Would this be the culprit? I am going to doublecheck other resistor values. I had purchased this board from a member here, had come mostly already populated.
You got it!
That's not possible. Let's do it that way: report the voltages you get on the schematic.
My guess is that R8 or R10 have an incorrect value.
After going through all the resistors indeed I found R9, R10 had incorrect values. On the board are 30k resistors, where BOM calls for 47k. Also R13 is 27k, where BOM calls for 39k.
The measurements I posted for R10 and R8 were with multimeter probes clipped on either side of the resistor. Sorry I guess I misunderstood. Measuring against gnd (tp3) for each resistor, I get following readings for either side of each resistor. Sorry if still incorrect.
R10, 307V, 368v
R8 - 307v, 25v
I will get an order going for correct value resistors. I also wanted to ask about CX1, BOM calls for 10nf or 100nf 1000V cap here, what's on the board is unmarked. Should I swap this out?