Tag: 300B Amplifier

Gassy nightmare

Ok, title isn’t very appealing. I know, however it truly reflects what I experienced over the past few days. Let me clarify before I start: a gassy tube developed a nice fault-finding journey. You always learn a lot from all this, for sure. Luckily, due to the holiday season, I had the time to work on this. I’d rather have spent it otherwise, but my 300B amplifier was dead.

6BY5-GA damper

I had a gassy 6BY5-GA damper rectifier tube. I should have changed it, but I was lazy and I paid the price for it. When turned on the 300B HT power supply, it will flash blue for a sec or two and the gas ionisation will disappear after a minute or two when tube was warm enough and operated as expected. Problem was that my stash of these damper tubes was up in the storage, who knows where.

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01a Low Gain DHT preamp (finished)

It was a half-hour job to mod my Mule preamp by swapping out the UX-120 and introducing the 01a (CX-301a in fact), add the resistor divider to deliver low gain on this preamp:

01a mule

I set the valves at 115V/4mA. The Source Followers are running at 15mA and the resistor divider is actually 100KΩ/33KΩ (Kiwame) which is what I had at hand. It worked like a charm.

The 01a is driving my Slagle AVC into the 300B amplifier. I can say I can hear now the subtlety of the thoriated-tungsten filaments of the 01a with the extra level of gain I was hoping for. The amp sounds very dynamic at high level and with the clarity of the 01a.

01a mule

This is indeed a clear winner. I will leave this preamp on my system for a while to confirm further impressions. For now, I just simply love it.

300B/845 Amp: Driver Options

For more than 6 months I’ve been playing with a lot of satisfaction my 300B amplifier with the 47 driver. Since then, I’ve made 2 modifications:

  1. Major one was the introduction of my Open-Baffle (OB) speakers. These are the “Liionidas” based on the LiiAudio C-15 and W-15 drivers. I need to do a proper write-up of these, but have to admit their sound has taken my system to another level. They are simply superb. Detail and dynamics are beyond expectations. Nothing less to be expected from the designer (Leon Huijgen) :

Initial test of the Liionidas OB speakers

  1. Second modification was minor, I removed the Zobel network from the input LL7903 to minimise loading to the source. Also, after several tests I found that the impact in the HF resonant peak is minor as the source has some resistance (below 100Ω) which is beneficial. So the 300B amplifier looks like this now:

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300B SE Amp: 46 Driver (Part II)

The 300B amplifier journey continues as planned. A bit lower than expected, however small steps being made. Recently I mentioned about the 46 driver.

I made some progress on the LL7903 step-up transformer which is wired on 1:8.

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300B SE Amplifier Finished!

When everything was going to plan…

This build became one of the quickest and eventually the most painful from all, perhaps not really. However, it was very challenging in the end. I will tell you why in more detail. Yet, it has been a fantastic learning experience.

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300B SE Amp: build part IX (Layout)

Feedback was that more pictures were preferred. So here they are. I have little time, but slowly I will make progress I hope. The main chopping board is what is left now. Layout is tricky as have not enough space given the size of the OPTs 😀

Checking if the EML300B will fit in the cabinet
layout, I will have to drill a lot of M3 holes for the mounting hex standoffs!

300B SE Amp: build part VIII (Cap Boards)

I made a set of useful PCBs. They are intended to mount large (big really big) film capacitors: WIMA DC Link ones!

WIMA DC Link mounting boards + PIO cap board

I use the cost-effective 45μF/600V (MF Part No. DCP4I054507ID2KYSD) in many of my boards as the last capacitor in the filtering network. This is a 2 pin device, however when you go larger like the 80μF/900V (MF Part No. DCP4N058009JD4KYSD), this one has 4 pins and bigger size. The PCB for the later can also accommodate the smaller DC Link of 45μF/600V. The boards have turret or 2mm banana plug connections and an INS-1 Nixie indicator with its associated resistor. Finally a bleeder 3-5W resistor can be added.

The smaller board has the size of the Source Follower PCB. It can be mounted below it or can be used independently. Can fit a variety of PIO/Film capacitors for decoupling or for AC interstage coupling.

Speaking about the Source Follower PCB, I made also a new batch of PCBs as run out of the original ones. I made a minor modification and improvement by adding an LED indicator before the top MOSFET drain. This works in the same way as the gyrator Rev08 PCB. Can be used for normal operation or for A2 current source indicator. Also added an extra PIO/Film 100nF decoupling cap to be mounted under the PCB to decouple the high impedance node to the power supply:

SF PCB REv03 test

SF PCB Rev03: decoupling cap under board

300B SE Amp: build part VI (Fixed Bias Board)

And the fixed bias PCB is completed. All individual PCBs mounted over a ground plane PCB. It will be a stacked build. On top of this PCB, another one will hold the driver. Firstly the D3a in a hybrid mu-follower configuration:

PCB mounted, now to wire them

Nice dim glow from the HV LED array. Boards are working properly and the heatsinks good enough for 20mA idle per channel.

Tested and bandwidth of these source followers is nearly 10MHz with plenty of current drive at 20mA idle.

300B SE Amp: build part V (Fixed Bias)

Last year I developed a voltage reference using an HV LED. Unfortunately these devices from OSRAM seem to be discontinued. I managed to buy enough parts for my own use though. What is interesting from these LEDs is that the dynamic resistance is low. About 150Ω with good current, or between 300Ω to 500Ω.  Tempco is very low and with such a low dynamic resistance, they are great for creating a voltage reference with a stable CCS:

The LEDs are extremely bright and found that with a 1mA current are dim enough whilst retaining the stability needed. I have a cap multiplier arrangement and the LED array is fed by a stable CCS. Jumpers on the board allow bypassing LEDs and there is also the option to use a trimmer for variable voltage adjustment. Very handy for screen grid supply and phono stages. The reference voltage is extremely quiet with more than 100dB PSR.

OSRAN 67V LED curves

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