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Old 6th July 2004, 08:00 AM   #1
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Default New PP amp just completed - a few unusual features (long)

Dear All,

I finally completed my PP EL34 (triode) integrated stereo amp and it sounds OK. The amplifier topology is as follows:

1. IP selector switch;

2. Passive Baxandall bass and treble controls (I like the felxibility of tone controls and I'm no purist!);

3. Volume control;

4. 6AU6 preamp in triode mode, using g2 as the anode and with plate and g3 grounded (keeps the current down and provides very good shielding); this stage has an unbypassed cathode resistor to provide some negative current FB;

5. 6SL7 long-tail pair splitter, direct-coupled from the preamp, with a 6AU6 as a CCS in the tail (pentode connected); each cathode has a 1K resistor between it and the plate of the 6AU6 to assist in balancing and to provide some negative current FB;

6. 6SN7 differential amp driver, connected from the 6SL7 splitter through a step-network and with resistors from the plates of the 6SN7 to the plates of the 6SL7 to provide some shunt negative FB;

7. EL34s triode-connected, feeding into the output transformer; the plate load resistors for the 6SN7 driver are connected to the plates of EL34s, to provide shunt negative FB and reasonable damping factor (I calculate DF ~ 7.5); EL34 cathodes have separate 220 ohm 5W resistors, then a common 110 ohm 10W resistor to ground (per channel); each cathode is bypassed to ground with 220uF capacitor;

8. OPTs are no-name, locally made, of E-I construction, supposed to be able to handle 100w (which I doubt) but only called upon to handle 15w or so in this application;

9. Power supply uses an Amplimo toroid xfmr, nominally 340v 700mA, 50v 100mA (not used) and 6.3v 6.8A used for the rectifier heaters; rectifier is a hybrid bridge, using 6D22S damper diodes and MUR1400E diodes; the filter is of CLCLC design;

10. A second Amplimo toroid xfmr with two 6.3v 6.8A windings is used for the heaters of the left and right channels, with resistive center-tap set at +55v to avoid breakdown of the heater-cathode insulation in the 6SL7s.

Each stage has local FB, to make gain less dependent on tube age. There is no global FB, mainly because I don't trust the OPTs to behave themselves.

I had to do some trimming of the resistors in the plates of the splitter, in the step network between the splitter and driver and in the plates of the driver, to achieve good balance. This was mostly because the resistors I used were not of sufficiently close tolerance but I also suspect the 6SL7 and 6SN7 tubes were not closley matched.

With an integrated amp like this, especially using AC on all heaters, I expected hum to be a problem. However, I have been pleasantly surprised by the quietness of this amp.

I can't hear any hum or noise at all and the tone of the amp sounds very good to me, despite using Sovtek tubes throughout (except for the damper diodes) and non-pedigree OPTs. The speakers are Yamaha NS-83980 3-way units of sensitivity 89dB. Maybe one day I'll treat myself to a pair of Lundahl OPTs, get some better speakers and try some tube rolling (when I have some spare cash) but, until then, I'm really happy with my result.

Thanks for your patience if you've read all of this!

Regards,

Ray
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Old 6th July 2004, 11:45 AM   #2
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What sort of p-p anode load do your no-name transformers provide? B+?

One thing you might try in order to get some feedback around the OPTs is to ground the 4 ohm tap, then connect the EL34 cathodes to the zero and 16 ohm taps. With your bias setup, you could run the 110 ohm resistor from the 4 ohm tap to ground, bypassing it with a big cap, then put the 220 ohm resistors between the cathodes and the xfrmer taps, with big caps across each of the resistors.

Congrats on getting the project up and running!
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Old 7th July 2004, 04:48 AM   #3
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This pair of OPTs wan't made sepcially for me, I just bought it off the shelf but it's locally made (in Jakarta). The OPT unfortunately has taps only for 4 ohm and 8 ohm; if it had a 16 ohm tap, then I would certainly try your suggestion and get a few dBs of neg. feedback that way. I'm sure it would improve the damping factor, if nothing else (altough the bass doesn't sound too boomy).

I'm using the 4 ohm tap for my 8 ohm speakers. The reason is that, when I measure the primary-to-secondary ratio (by sticking the mains across the primary end-to-end and measuring the secondary voltages), it appeared that this is a 3,500 ohm OPT, serving 4 or 8 ohm loads. So, I figured that using the 4 ohm tap would give me a load of 7,500 ohms, that's why I set the bias resistors of the EL34s to give the equivalent of 460 ohms per tube, which is roughly midway between the two operating points recommended by Mullard for 5,000 ohm and 10,000 ohm loads.

B+ for the OP tubes is taken after the 5 Henry choke and measures 390v. That's a bit low and I'm thinking of using an all SS bridge and putting the 6D22Ss (in parallel) after the first (50uF) smoothing capacitor and before the 5H choke, so I still get a slow start. This would have the effect, I guess, of increasing the effective DCR of the choke from 40 ohms to something like 80 ohms (please correct me if I'm wrong) but the much lower forward resistance of the SS diodes in the bridge would give me a bit more voltage, maybe around 420v which would be ideal. If I needed more I could increase the size of the smoothing C but I have to be careful not to exceed the 700mA rating of the power xfmr secondary. It gets quite warm enough as it is!

Regards,

Ray
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Old 7th July 2004, 12:18 PM   #4
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At 420V, you should be able to manage a tad more than 20W, which ain't bad. Good, solid design approach you've taken!
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Old 9th July 2004, 02:41 AM   #5
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TYFYKW! (thank you for your kind words).

One thing of interest (to me, anyway): I did a bit of tube-rolling last night (yes, I admit it!). That is, I swapped around the pair of GE 6AU6s I was using as the input tubes with the RCA 6AU6s I was using as current sinks for the 6SL7 LTP splitter. The difference in sound was astounding: much more gain and clearer sound with the RCA tubes in the input stage. I guess that means either that the RCA tubes run better with screen grid as anode that the GE ones, or that the GE tubes are in poorer condition. Whatever the reason is, I'm pleased with the improved result.
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