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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 13 Jul 2019, 07:59 
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Location: australia
Thanks will try some changes. the 6sn7 heaters have a centre tap to signal ground so that should be ok. one i need to check is if the 5v heater regs are oscillating as per Matt’s previous suggestion. I need to borrow a scope for that. The buzz almost sounds as though it is within the chassis. I noticed the rectifier socket bakelite material is a bit loose too so I will try another one i have.

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PostPosted: 13 Jul 2019, 16:21 
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For the regulators its unlikely they will oscillate.
I thought we've tired making them arround 4v so they have enough head room to regulate the voltage and this means at 4v there shouldn't be noise on regulators.
But this didn't make any diffrence.

You still need 0.1uF on the adj pin, vin and vout pin of the regulator.
This has low impedance at hf and will help reduce the hf noise if any. But this change will unlikely fix the noise.


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PostPosted: 14 Jul 2019, 06:48 
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ILoveHiFi wrote:
I thought we've tired making them arround 4v so they have enough head room to regulate the voltage and this means at 4v there shouldn't be noise on regulators.

:blush: first of all i’ll get the red face out of the way. i was so happy achieving 5v from the regs I didn’t think to look for this as a cause of the ripple although i think i mentioned a change in the pitch of the ripple at that point. just before as per your suggestion ILHF i reduced the filament voltage to 4.6 v, that’s as far as i could go. lo and behold ripple was about 2mv at the speaker output. (not sure if that’s the best place to measure it). thinking this might be a bit low for the filaments i played around to see how far i could increase it before it exceeds the 12mv i was aiming for for the amp from the power supply design guideline. that seems to be about 4.7 v for around 8 mv ripple. close up to the speaker I could still hear this but not from the sofa.

While i am thrilled at the result I am still facing having to get a filament transformer that will handle the job of powering the bridge rectifiers and the regulators without oscillation if that’s what is occurring. maybe the 2.5 amp 6.3v hammond will be ok. mine is only 6v @2A. Suggestions appreciated on any alternatives. Thx Koichi hi for reminding me of this.

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PostPosted: 14 Jul 2019, 06:53 
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ILoveHiFi wrote:

You still need 0.1uF on the adj pin, vin and vout pin of the regulator.
This has low impedance at hf and will help reduce the hf noise if any. But this change will unlikely fix the noise.


Koichi, where would i solder the other end of this caps, to the floating ground side? I could give a try to see if any effect but the reg tag strip is getting crowded, i should have put it on perf board but that would be something new for me.

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PostPosted: 17 Jul 2019, 14:58 
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If the heater is biased up to reduce the cathode to heater voltage.
Then photos shown is how you do it, any one of the 3 ways will work.

PIC 3 reperesents pic1 and 2, is littreatley the same schematic with same effects,
But expressed in a more general way

Otherwise if your connecting to 300b direct heaters, then the PSU ground and AMP ground is fully isolated and not connected.


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PostPosted: 17 Jul 2019, 16:54 
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Thanks Koichi. The buzz is almost inaudible now and the amp is sounding great to me now so I might leave things be until I procure a filament transformer that can do the job as intended. I have emailed Hammond to get an opinion on whether the one I have (266LA12) is up to the task or is perhaps faulty and should be replaced but no word as yet. The only other change I have made is to remove the 100 ohm resistors in the secondary wires, as this didn't improve the buzz anyway. I might have mentioned it but I put one of the 100 ohm resistors and spare 39 uF capacitor for additional filtering of the b+ to try and reduce the hum (this was before I found the regulators tone making the noise so made no difference to the buzz). I may return things as they were to be as faithful as possible to the original. The b+ is about 420v now. I replaced the dropping resistor with a 10k to the 6sn7 supply so this is 300v now.

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PostPosted: 17 Jul 2019, 19:10 
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Joined: 06 Jun 2008, 18:23
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ILoveHiFi wrote:
If the heater is biased up to reduce the cathode to heater voltage.
Then photos shown is how you do it, any one of the 3 ways will work.

PIC 3 reperesents pic1 and 2, is littreatley the same schematic with same effects,
But expressed in a more general way

Otherwise if your connecting to 300b direct heaters, then the PSU ground and AMP ground is fully isolated and not connected.

Etch A Sketch?

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PostPosted: 17 Jul 2019, 20:10 
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A very cheap alternative could be to use this one
Searching buckboost converters, these will give you a regulated output votlage, regardless of input voltage. can get 5v out even if 3v input or 8v input
But you need to buy fancy shpping if you don't want to wait up to 3 months to recive it

https://www.aliexpress.com/item/3295261 ... c112366e7c

https://www.aliexpress.com/item/3262779 ... c112366e7c


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PostPosted: 24 Jul 2019, 00:19 
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Location: australia
Suncalc wrote:

It could be that the 2A secondary is marginal and a 2.5A secondary is really required to prevent startup problems. Hammond transformers tend to have higher secondary resistance than most other power transformers and this could be contributing to your issues.


Hammond have been very patient and timely looking at this problem with me. According to the secondary unloaded voltage of about 6.5v the unit is working as expected. it rating perhaps is not enough for the task. I understand mwhouston used the 266L12 with a 2.5A rating with success. Any comments would be appreciated.

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PostPosted: 24 Jul 2019, 02:31 
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For up to 25C room temperatures, its generally very reliable to run the transformers up to 70% of maximum ratings.
They will get hot arround 55-70C. But if you look at specs of raw material they have a very very long life even at this high temperatures
Most insulation rated for above 125C and fancy enamled copper wire is 180C. Don't know about the core though.

I always run my transformers cooking hot and they have been reliable for long period of use.


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