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Old 2nd October 2008, 04:52 PM   #1
jazzbo is offline jazzbo  Canada
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Default Hashimoto A-105 - 6S4A

6S4A vs. 6S4A regulated plate voltage frequency response with the A-105. Wasn't expecting this much difference...
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Old 2nd October 2008, 04:55 PM   #2
jazzbo is offline jazzbo  Canada
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Default 5687 with various secondary loadings

20 -27K looks good but the loss of gain is too high, about 13V/V with a 20K load.
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Old 2nd October 2008, 04:57 PM   #3
jazzbo is offline jazzbo  Canada
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Default ECC99 with 20K secondary load

Still sufficient gain and the peaking is manageable... Gain down to ~18V/V.
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Old 3rd October 2008, 09:28 AM   #4
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Did you listen to the alternatives above?

I'm curious because I'm using the 6S4 as a driver and indeed as an output tube in PPP.

Should I be expecting bass rolloff?

Andy
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Old 3rd October 2008, 11:11 AM   #5
jazzbo is offline jazzbo  Canada
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I'm still testing driver candidates, so haven't listened to anything yet. The amp will be an IT coupled 300B SE. The 6C45Pi is the last tube in the list, though I may go and fiddle with the 46 again.

So far the successful candidates are the 6S4A, 12BH7 and the ECC99 with loaded secondary. I would like to stay with a tube that has sufficient gain for a two stage amp.

The 6S4A is down 1dB at 30Hz through the IT. Will you notice this? Possibly, depending on your speaker response et al. I played with bypass cap values on the ECC99 and found that you can tweak the low end response somewhat by using smaller than ideal values, which produces a low frequency resonance and a slightly tilted up low end response. I haven't tried that with the 6S4A.
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Old 3rd October 2008, 02:20 PM   #6
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If you want the best sounding driver for the 300b, then in my opinion - and that of several other listeners in a shootout with 26, 46 and 2a3 - it's the 10Y. The 46 probably second best.

Andy
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Old 3rd October 2008, 03:58 PM   #7
kevinkr is offline kevinkr  United States
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I'm wondering whether using a series RC network in parallel with either the primary (reducing HF load and hence gain) or secondary (here as a load) might be a worthwhile experiment.

I have also heard in some cases lately that people are using tubes with much higher rp's than would normally be the case (for a given primary impedance) to drive IT transformers precisely because of the ultrasonic rise in response. (A number of people in my local tube group have been using IT for a long time and have used this approach successfully.) It does place rather onerous demands on primary inductance and transformer construction particularly to reduce interwinding capacitances.

Some additional R in series with the primary will also reduce the ultrasonic resonance at the expense of overall gain as will higher winding resistances.
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Old 3rd October 2008, 04:13 PM   #8
jazzbo is offline jazzbo  Canada
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Kevin, that's why I'm leaning toward the 6S4A (rp = 3.7K) and 12BH7 (rp = 5.3K), little or no HF peaking and sufficient gain for my purposes. As you state, any type of loading will reduce gain. The flattest response from let's say 40Hz - 10kHz is the ECC99 so I will continue to play with secondary loading and try a series RC across the secondary.

Have any of your IT users commented on the sound of a higher rp tube versus lower rp with it's HF peaking? Does it tend to harden the sound? I may try something in the 6 -7K range at a lower current to maximize the inductance of the IT.

I've got a 6C45Pi in circuit now. I believe the rp is <1K so I may try low current (15 mA or so) and parallel the windings. This will reduce the inductance of the A-105 to ~15H. We'll see what happens.
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Old 3rd October 2008, 04:56 PM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by jazzbo
Kevin, that's why I'm leaning toward the 6S4A (rp = 3.7K) and 12BH7 (rp = 5.3K), little or no HF peaking and sufficient gain for my purposes. As you state, any type of loading will reduce gain. The flattest response from let's say 40Hz - 10kHz is the ECC99 so I will continue to play with secondary loading and try a series RC across the secondary.

Have any of your IT users commented on the sound of a higher rp tube versus lower rp with it's HF peaking? Does it tend to harden the sound? I may try something in the 6 -7K range at a lower current to maximize the inductance of the IT.

I've got a 6C45Pi in circuit now. I believe the rp is <1K so I may try low current (15 mA or so) and parallel the windings. This will reduce the inductance of the A-105 to ~15H. We'll see what happens.
Sounds like you are proceeding down the same line of thought.. Haven't heard anything about hardening of the sound using higher rp tubes to drive a given transformer.

Should be interesting to see what happens with the 6C45pi and parallel windings. I would think that the 5842 would be pretty reasonable in this configuration as well and at about the same current.

Your experiments are quite interesting to me because I am trying to decide whether I will use an IT in my next amplifier which is contemplated to be VT4C running in A2.. My last IT coupled design wasn't entirely to my liking although my client liked it well enough.. (Tango iron)
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Old 3rd October 2008, 05:21 PM   #10
jazzbo is offline jazzbo  Canada
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Kevin, I meant does the tube with the lower rp and HF peaking harden the sound...

I've read (RDH4?) that loading the secondary increases transformer distortion. kmaier says no, it lowers it. Any opinion on this?
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