Portaflex Build (First Build and Pics)

[GregS]

I just completed the Ampeg Portaflex project from the plans in Kevin O'Connor's book "The Ultimate Tone 3". This was my first build and quite a learning experience. I have attached a few pictures in this thread. I have yet to do the labelling of the chassis. I have a the cage for the top to protect the transformers and tubes. I don't have it on in the pictures.

Generally, I think the build turned out well. I drilled all the chassis holes with a hand drill and step drill bits and I think it resulted in a reasonably neat job. I would have prefered my wiring to have been neater and more orderly but hopefully I'll improve on this with time.

I faced a few challenges with the build. First, the large filter caps and power resistors wouldn't fit across a single eyelet board. I doubled the boards together with pieces of wood underneath to solve this problem as can be seen from the pictures. However, it made everything physically a lot more awkward.
Next, I was uncertain at the time I ordered the parts if I should use linear or audio taper pots so I ordered a set of both. Unfortunately, the linear had a smaller hole size than the audio so I was concerned if I drilled the larger hole sizes for the audio and they weren't right, I would not be able to mount the linears properly afterwards. I decided to install the linears and try them first as I could always re-drill larger holes for the audios if the linears didn't work well.
The biggest dilema I had was with the circuit grounding. The plans have a galactic or star grounding scheme with one connection point to the chassis through a ground lift switch. This grounding scheme depends on the use of insulated jacks. The jacks I ordered were non-insulated so I had a problem with implementing this correctly. I could not find the Switchcraft jacks that were recommended on the Antique Electronic Supply site in an insulated design - Where do others go for good insulated jacks? As a result, all circuit grounds go through the lift switch to a lug near the power supply except the speaker output jacks and OT ground leads which ground to the chassis at that point. I also wired the speaker output jack and OT grounds to a ground lug near the jacks to ensure a good ground connection - so I actually have 3 circuit grounding points. I decided to do it this way and hope for the best.

THE FINAL RESULTS:
I did the standard first power up tests without tubes and then with tubes taking voltage readings at all the relevant points and everything seemed to check out ok.
I then tried it out with the bass. It sounds really good as I was hoping it would. It has good tone, impressive volume for the rated power output, and the hum is not bad. There are a few glitches though:

1. The pots seem to have most of their effect in the first 1/3 of their travel - perhaps the audio taper might even this out better?

2. When circuit grounds are lifted with the ground lift switch there is significantly more hum and no instrument sound. I'm not that surprised as I wasn't able to do this properly as described above. However I can live with this - I just won't use it.

3. It seems to pick up and amplify mechanical vibration to an extent. If I tap my finger on the chassis at various points it will be amplified through the speaker. Is this normal? What causes this? It is a problem if the amplifier is sitting on the cab and turned up loud which will likely be the case in a gig. Right now I don't have any rubber feet on it to isolate it from the cabinet vibration so maybe that would help?

Hopefully I haven't put anyone to sleep whose read this far. Thanks for your patience if you have.

Greg

[GregS]

More pictures.

[GregS]

One final one. Not sure how to do them all in one post.

Greg

[GregS]

It seems to pick up and amplify mechanical vibration to an extent. If I tap my finger on the chassis at various points it will be amplified through the speaker. Is this normal? What causes this?

It turns out to be a microphonic pre-amp tube. Replaced it and no longer have the problem. I'm glad it wasn't something more serious.

Greg

[tubeswell]

2. When circuit grounds are lifted with the ground lift switch there is significantly more hum and no instrument sound. I'm not that surprised as I wasn't able to do this properly as described above. However I can live with this - I just won't use it.
Greg

1. Yep audio taper pots are preferred.

2. If you disconnect the ground return path, the amp won't work. The quietest way of grounding I have found is a split-ground system (a.k.a. "Hoffman style" grounding), where all the pre-amp grounds (including the ground side of the pre-amp filter/decoupling cap go) each via separate wires, to the ground lug on the input jack, and all the power amp grounds (including the screen node filter cap ground and the reservoir cap ground) and the output tube cathode ground, and output tube grid leak resistor ground go to one of the PT bolts. You can either put the heater CT with the output tube grounds, or if your amp is cathode biased, you can run the heater CT to a/the output tube cathode pin

[GregS]

I think I'll try my audio taper pots. Although it is tempting to leave it alone as it's working well otherwise This is probably worth doing.

With regards to the ground lift function when the ground is "lifted" the ground return path goes through a ceramic disk capacitor and resistor in series before going to the chassis lug so it is not entirely disconnected. Although I'm not going to bother with it any more in this build, I'm curious if it would work if there were only one circuit ground point to the chassis.

Tubeswell, thanks for the info on the alternate ground approach. I'll probably try that in my next build. You mention two chassis ground points, the input jack ground lug and the PT bolt. Does this approach require the use of insulated speaker output jacks as there would be a third ground point with a non-insulated jack?

Thanks,

Greg

[loudthud]

There is a mistake Kevin tends to make on some of his Ampeg schematics. There is a resistor that goes between the center terminal of the Bass control and the center terminal of the Treble control. The connection to the next stage's grid should go to the treble control side. Otherwise, the Treble control won't have much range.

[GregS]

There is a mistake Kevin tends to make on some of his Ampeg schematics. There is a resistor that goes between the center terminal of the Bass control and the center terminal of the Treble control. The connection to the next stage's grid should go to the treble control side. Otherwise, the Treble control won't have much range.

Interesting. This explains why my treble control has almost no effect. There is a 120K resistor in the schematic between the centre terminals with the connection to the next stage's grid on the bass side. I will change that to the treble side and see what happens. Thanks for pointing that out.

Greg

[loudthud]

I cracked open TUT3 last night and looked at Kevin's layout. In the layout he connected the next stage to the treble control (where it should be) but left the 120K resistor (the one 'tween treb and bass) off completely. It must have been late the night he did that

Your layout is a little disorganized but it should run just fine. If you follow Kevin's grounding, your amp will be studio quiet. Hoffman's grounding works almost as well. Hoffman sells the washers to insulate chassis mount phone jacks from ground. I prefer plastic jacks on inputs and metal jacks on speakers. I've had good luck just having the one chassis connection at the speaker jacks and insulating everything else.

[GregS]

Yeah, I noticed the 120k resistor was missing from the layout diagram. I decided to wire it according to the schematic instead. I re-wired it so that the treble side connects to the next stage (as you mentioned) and now the treble control works well.

I didn't exactly follow Kevin's grounding as none of the jacks I used are insulated. Aside from that I followed his grounding. Trying it out at home noise is not a problem. The real test will come when I try it at the rehearsal place with my regular bass. The amp I've been using for years up to now is extremely noisy in these conditions.

Thanks for the info on where to get the insulating washers.

Greg

[H Chandler]

What did you use for transformers?

[GregS]

Hammond 270FX for power transformer.
Hammond 1650F for output transformer.

Greg

[Jwm]

Hello, I'm just starting this same project. I'm a bit of a newbie to electronics in general but I'm having a seasoned tube-amp builder help me. Any advice?

-Jeremy

[tubeswell]

Hello, I'm just starting this same project. I'm a bit of a newbie to electronics in general but I'm having a seasoned tube-amp builder help me. Any advice?

-Jeremy

Hi Jeremy and welcome to the forum

You'll have better luck with advice if you start a new thread (instead of jumping on to an old one) - 2CW of advice

[GregS]

Hello, I'm just starting this same project. I'm a bit of a newbie to electronics in general but I'm having a seasoned tube-amp builder help me. Any advice?

Assuming your working from the plans in TUT3 the significant points of advice are contained in this thread.

1. If you are not using insulating jacks don't attempt the grounding scheme Kevin O'Connor suggests. Go with the Hoffman scheme. I tried that with the next build I did (without insulating jacks) and the results were quite good.

2. Take note of the mistake in the schematic that Loudthud pointed out in this thread that causes the treble control to be ineffective. Wire it as he suggests.

3. Use audio taper pots.

4. The project suggests using a 270 Ohm cathode resistor for the power tubes instead of the 250 Ohm that Ampeg used if I recall correctly. The 270 Ohm option increases tube life by not working them so hard but limits headroom slightly. I went with the 270 Ohm option but find I don't quite have enough headroom rehearsing with a band. If I were to do it again I would opt for more headroom / shorter tube life. Although it isn't a big deal to change the resistor I never seem to find the time or opportunity to do it. Other projects have more priority.

Greg