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#26 pre amp
#26 pre amp
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Old 3rd December 2012, 04:19 PM   #2241
Magz is offline Magz  United States
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I'm happy enough with the bias battery under the input transformer that I've permanently changed the preamp, removing the input capacitors and grid bias resistors, and relocating the batteries to where the input caps were previously.

It sounds much better this way, and the signal path is a little bit neater as well. Pic below.
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File Type: jpg 26 Preamp Battery under L1676.jpg (153.0 KB, 652 views)
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Old 3rd December 2012, 04:20 PM   #2242
UKToecutter is offline UKToecutter  United Kingdom
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#26 pre amp
Beautiful............... :-)
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Old 3rd December 2012, 10:04 PM   #2243
Vbenonisen is offline Vbenonisen  Norway
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Hello, just a question to Magz.

I have a 26 preamp with input capacitors and battery bias and just so happen to have two LL1676 input transformers laying around. I probably need to have them wired in 1:1 but how do you calculate the RC network and what is the purpose of the battery bypass?

Very nice work on your preamp by the way...
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Old 3rd December 2012, 11:08 PM   #2244
Magz is offline Magz  United States
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Quote:
Originally Posted by Vbenonisen View Post
Hello, just a question to Magz.

I have a 26 preamp with input capacitors and battery bias and just so happen to have two LL1676 input transformers laying around. I probably need to have them wired in 1:1 but how do you calculate the RC network and what is the purpose of the battery bypass?

Very nice work on your preamp by the way...
Mine are wired 2:1 parallel/parallel and that is perfect for use with the autoformer outputs/volume controls. How you wire them determines the RC network you need, so parallel/parallel, parallel/serial, serial/parallel and serial/serial all have somewhat different resonances and require somewhat different networks. I used a function generator and oscilloscope to determine what gave the best square wave performance; the easiest method is to wire the cap to a pot and vary the pot to get the best setting, then replace the pot with a resistor. I split the resistor value in two and used an RCR setup to isolate the transformer somewhat from the cap.

I put in a small bypass on the battery (.01uf polystyrene) in case there is any HF noise that needs to be bypassed to ground and may be stopped by the battery.
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Old 4th December 2012, 01:22 AM   #2245
Vbenonisen is offline Vbenonisen  Norway
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I thought that it was not recommended to use the secondaries in parallel on the LL1676 ("frequency abberations" according to KevinC) but I'll guess it depends on the expertise of the user.

I have no scope but perhaps you can recommend some starter values on the RC network just to try? Or maybe it's best to use nothing just to hear if the transformer has any potential of bettering my Duelund Cast?
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Old 4th December 2012, 02:12 AM   #2246
Magz is offline Magz  United States
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Quote:
Originally Posted by Vbenonisen View Post
I thought that it was not recommended to use the secondaries in parallel on the LL1676 ("frequency abberations" according to KevinC) but I'll guess it depends on the expertise of the user.

I have no scope but perhaps you can recommend some starter values on the RC network just to try? Or maybe it's best to use nothing just to hear if the transformer has any potential of bettering my Duelund Cast?
I know of Kevin's thoughts, but my scope showed the 2:1 (parallel) actually had less HF ringing than 4:2 (serial) for a 2:1 connection. I was using a CAST 0.22, but in addition to the LL1676; the transformer alone is definitely better.

I'll tell you what..I have a spare LL1676. Tell me how you want to hook it up, and I'll scope it and recommend an RC.
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Old 4th December 2012, 02:22 PM   #2247
Magz is offline Magz  United States
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Vbenonisen,

I had some time this morning, so I wired up the LL1676 in 1:1 as 1+1:2||2. The RC network that worked well was 2nF and 2Kohm.

2K square wave scope shots below: first is the LL1676 with no RC, second is with 2nF+2Kohm.
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File Type: jpg LL1676 1to1 with no snubber, 2kSQWV.jpg (238.3 KB, 554 views)
File Type: jpg LL1676 1to1 with 2nF, 2k snubber. 2kSQWV.jpg (224.4 KB, 496 views)
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Old 4th December 2012, 04:27 PM   #2248
Vbenonisen is offline Vbenonisen  Norway
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Thank you Magz! Much appreciated. It sure is some helpful people on this thread.
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Old 5th December 2012, 07:17 AM   #2249
Richard is offline Richard  Australia
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Quote:
In any case, the transformer wouldn't be damaged by the DC, the core would saturate, which can be remedied by passing an AC signal at the saturation level...demagnetizing, in other words.
Thank you for the feedback Magz. I'll give it a shot.

Cheers,

Rich
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Old 6th December 2012, 11:50 PM   #2250
Vbenonisen is offline Vbenonisen  Norway
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Default RC network on VR tubes?

Is there somebody out there who knows what's the appropriate RC network to shunt a 0D3 VR-tube with to get a flat power supply impedance curve?

V
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