This is a surprise !
I have thought that the windings are equal like MacIntosh uses.
Can you show the schematic you used ?
EDCOR Electronics Corporation. GXSE Series Output Transformers
Edcor sells directly and with reasonable shipping cost.
Ofcourse we in EU must add the VAT.
Havn't quite mastered the method of computer manipulation so not able yet to upload the picture., but the essence is per the Quad drawing with the cathode cotaining the secodary winding.
Hi,
Costs are for the:
10W $155.57 Aprox £101.032
15W $ 193.30 / Aprox £ 125.54
No VAT added!
Just waiting for Paypal to clear the funds so I can get a sale price.
The above is with aprox shipping.
I will be using the 15W and power TX as discussed.
🙂
Regards
M. Gregg
Last edited:
I think I am following this if you could post a schematic and Toprepairman could agree / disagree.
I don't have a schematic of the Quad amp he refers to the 2 is pp?😕
Regards
M. Gregg
Last edited:
I have used the secondary of OPT to create NFB like this:
You have to check the phasing of the secondary winding so that the feedback will be negative. If it were positive, the amplifier would propably oscillate.
To get some local feedback for 12AX7, you should leave the 220 uF cathode bypass capacitor away. The the distortion of voltage amplifier section will be lower.
An externally hosted image should be here but it was not working when we last tested it.
You have to check the phasing of the secondary winding so that the feedback will be negative. If it were positive, the amplifier would propably oscillate.
To get some local feedback for 12AX7, you should leave the 220 uF cathode bypass capacitor away. The the distortion of voltage amplifier section will be lower.
I should clarify the above costs are 2x O/P Tx and 1x power tx plus Approx shipping.
No vat is added!
Regards
M. Gregg
No vat is added!
Regards
M. Gregg
What voltage is expected on the O/P "speaker" terminals? ( I know its low due to Volt drop across the tube)
I have seen this type of circuit before just can't remember where!
Quote: To get some local feedback for 12AX7, you should leave the 220 uF cathode bypass capacitor away
The gain will be reduced on the first stage? Would you still have feedback from the output Tx as before?
Regards
M. Gregg
Last edited:
Do you mean what output power ? I think the power level will not differ a lot from earlier "UL with global feedback"-circuit.
No. That is no more possible. We have now the secondary phased to give NFB to the cathode of the output tube. If we now connected the signal from sencondary to the first tubes's cathode too, this would be positive feedback.
We could connect it in series with the grid of 12AX7, but then the gain of the whole amplifier would depend on the impedance of the source device.
This alternative requires a bit more thinking before I can recommend it.
If you are more interested in QUAD, below is one link ( 5 pages )
Quad II Classic monoblock power amplifier | Stereophile.com
I am refering to the voltage drop across the Tx output winding / how much current will flow through the speaker coil and voltage drop across it.
You will be able to measure the bias voltage across the speaker terminals.
I am looking at the safety factor should the O/P Tx secondary go open circuit.
Regards
M. Gregg
Last edited:
The DC-resistance of the secondary is very low. Less than 1 ohm. The DC-current thru it is the cathode current of EL34, i.e. some 100 mA.
Thank's
I had to ask the question, because if I didn't someone else would.🙂
So it looks like a few weeks to get the parts and start building!
Do you have an exact value of bias current for the EL34? You say 100mA is that the actual value!
Sorry from your previous post 113mA.
Kind regards
M. Gregg
Last edited:
I can not say what the exact anode current (+ screen current) value will be. It depends a lot about the supply voltage you finaly get from your power supply. Therefore you must adjust the cathode resistor when the amplifier will be ready for powered up for the first time.
The values I have given are based on the desing calculations and the data given by tube data sheets. The main thing is that the total current is adjusted to such level that the maximum permitted anode dissipation ( 25 W ) will not be exceeded. And the only way to do this is to fine tune the value of the cathode resistor of EL34.
The values I have given are based on the desing calculations and the data given by tube data sheets. The main thing is that the total current is adjusted to such level that the maximum permitted anode dissipation ( 25 W ) will not be exceeded. And the only way to do this is to fine tune the value of the cathode resistor of EL34.
With this arrangement the speaker terminals will be elevated some 20V relative to GND. If a speaker wire is accidentally touching gnd, the tube will go "poof".
Better put the OPT speaker winding below the cathode resistor and bypass cap.
Or, connect only the bypass cap to the speaker winding, as in the Simple SE
This eliminates any DC through the speaker circuit.
Svein
I agree.
Below is this modification done.
An externally hosted image should be here but it was not working when we last tested it.
Good morning.
Agreed with the last circuit. it is as I had arrived at, with the exception of only being in pentode mode, simply as a result of not having an ultralinear transformer.
I had fitted both an octal socket for trying EL34,EL37, 6L6, 6V6 etc and a B9A for EL84.
Agreed with the last circuit. it is as I had arrived at, with the exception of only being in pentode mode, simply as a result of not having an ultralinear transformer.
I had fitted both an octal socket for trying EL34,EL37, 6L6, 6V6 etc and a B9A for EL84.
Have you tried this circuit ? (you have an O/P Tx.)
What are your thoughts on the DC component across the speaker?
OK we are only looking at 50mA. Do we gain anything with the cathode current through the O/P winding with so low a turns ratio?
Regards
M. Gregg
Owing to the very low DC resistance of the OPT secondary there is virtually no DC voltage across the speaker (maybe a few millivolts). If you want to avoid that then simply connect the 120R cathode resistor direct to ground.
Lots to gain with this arrangement.
The circuit works very well.
I lent the amp to a 'golden ears' freind who's still raving.....
Lots to gain with this arrangement.
The circuit works very well.
I lent the amp to a 'golden ears' freind who's still raving.....
So the values of B+ are as stated 250-300V with 120 Ohm cathode resistor.
Quote: I lent the amp to a 'golden ears' freind who's still raving
When did you build it? 😀
Regards
M. Gregg
- Status
- Not open for further replies.