New 12w Liverpool build

[dblgun]

Having finished up helping some friends with their Express clones I'm finally able to get around to working on my Liverpool project. I'm using a Thordarson O/T and Hammond P/T. This will be as conventional a build as I can pull off with the limited Liverpool references available. I plan to use two ECL82'S in the power section for 10-12 watts.

[dblgun]

Only a small amount of apparent progress today.

[dblgun]

I had some more time to work on the amp today and made some good progress. I'm planning on getting it powered up tomorrow and I'll see where I am with the 6BM8's.

[dblgun]

Put power to her this morning and all went well. @wiliam vogel was correct that it required a larger cathode resistor to get it biased. Pretty happy with results. The pair of Valvo 6BM8's were well matched and a trio of Sylvania 12AX7's were solid as usual.

[dblgun]

The amp sounds good and is more sedate than my Express clone. I think they will pair well and am going to A/B them tomorrow. I think the 6BM8's were a good choice and am glad I could repurpose the O/T into this amp. The amp is biased at about 98% of 7w with 355v plate/cathode. Probably the lowest volume Trainwreck clone I've played. I'm going to live with it for a week or so before changing anything but think I may play with some values in the PI to reduce the voltages a bit.

[joeboo88]

Wow nice job!
I like your boards, where did they come from?
Your cabinets are very nice too.

I built a 6f6 Express, it’s amazing how loud these smaller tubes sound in these circuits

I have 2 sets of these 6bm8 tubes, I think I should copy you but build a small
Rocket. I have the iron as well. What size cathode resistor did you use?

Again great job

[dblgun]

Hey Joe,
The boards are very similar or the same as first used by KF in his builds, They are cut from a piece of Vector board commonly available from electronics suppliers like Mouser, Digikey etc. The terminals (flea clips) are specific to the board perforations and are also available from these same vendors. Let me know if you need the parts numbers and I'll dig them up for you.

The amp has a 10w 2k wirewound cathode resistor for each 6BM8. It could have easily shared a 1k between the two and been fine. I could and should have used a P/T with less voltage and the amp biased with less cathode voltage and less resistor. Something 260-0-260 to 270-0-270 would work great. The voltages in this build are more on par with an Express and some values, specifically in the phase inverter, needed to be adjusted.

As it is the plate to cathode voltage is 355v, with the cathode current at 20 and the plate current at 18.9. Bias is at 96% of max dissipation or 6.7w.

I think a 6BM8 Rocket would be great with possibly a 5V4GA as a rectifier.

[joeboo88]

thanks for the info. I looked on Mouser and saw the vector boards there, I didn't think these were still made.

I have an Edcor PT with 270 0 270, so I'll use that.

again nice build

[dblgun]

Glad you found them at Mouser. I have really taken a liking to the Vector board and flea clips. They are really easy to use once you get some time with them.

[pedro]

Hi. Great job on the new amp - looks great and i bet sounds wonderful.

I am making another express soon so i too would be interested in the part numbers for the vector boards and flea clips
Best,
Pete

[dblgun]

The board p/n comes in one piece and is enough to cut to proper size for both circuit and power boards. The clips are a bag of 100 and depending on your preferred layout plan to go through about 60. Expect to ruin a few before you get the hang of it.

Flea clips : Vector p/n T28/C Mouser p/n 574-T28/C

.062 Board : Vector p/n 170H48WE Mouser p/n 574-170H48WE

[pedro]

Many thanks dblgun
Do you have any sound clips of the new build ?

[dblgun]

Hey Pete,
I see you're in the U.K. Those Vector p/n's may be available from Farnell's there, I know I have bought them from Newark here in the U.S. in the past.

I do not have any sound clips of the amp yet. I A/B'd the amp and my Express last night and both completely overwhelmed my phones mic! The Express' ,as you know, can be brutally loud! I run the amps through a 4x12 of vintage Celestion Blackbacks which are very loud speakers. The new amp is much less loud than an Express but seems louder than the 2xEL84 Liverpool type builds I've been around. The higher voltages throughout the amp attribute to the higher perceived output I think. The beauty of 6BM8 is that they easily handle the higher voltages even though their max dissipation is only 7w's from the pentode. 6BQ5 and 6AQ5 would have been the options I considered with lower voltages. The option to add in the other cap and split the resistors adds additional options. This includes a bit more chime available towards the clean side of the guitars volume pot.

I'll try to get something recorded soon.

[Mark]

Nice looking build, so you didn’t us the triodes in the 6BM8.

You could have used the triodes in the PI stage, but it would mean a change of layout.

I built an adhoc 12 watt Liverpool amp. And I used a 1.8K choke resistor (to get the same sag in the power supply as the Express). I used a transformer with a 250-0-250vac secondary which gave me a good B+ voltage.

[dblgun]

Hi Mark,
I did not use the triodes in 6BM8's but have used them in the past including a GA5T clone that I did. I did not have any desire to stray too far from KF's original Liverpool layout other than where needed to facilitate the 6BM8's. I'm not familiar with what an adhoc amp is but would be curious. I used a 1.7k "choke" resistor in this amp which worked out well. In hindsight I should have used a 260-0-260 P/T I had but that will have to wait for a future build. I think the 6BM8's might be even better than they are in this build with voltages more in line with a Liverpool.